LIBRARY 

OF  THE 

UNIVERSITY  OF  CALIFORNIA. 

OP" 


RalfJL  6 


Received  ,  iqo 

Accession  No.     83070      •    Class  M?. 


THE 


PRINCIPLES 


OF 


THEORETICAL  CHEMISTRY, 


WITH  SPECIAL  REFERENCE  TO  THE 


CONSTITUTION  OF  CHEMICAL  COMPOUNDS. 


BY 

IRA    REMSEN, 

PROFESSOR  OF  CHEMISTRY  IN  THE  JOHNS  HOPKINS  UNIVERSITY. 


FIFTH  EDITION. 
THOROUGHLY    REVISED. 


LEA  BROTHERS  &  CO., 

PHILADELPHIA    AND    NEW    YORK. 

1897. 


Entered  according  to  the  Act  of  Congress,  in  the  year  1896,  by 

LEA    BROTHERS   &  CO., 
in  the  office  of  the  Librarian  of  Congress.     All  rights  reserved. 


DORNAN,  PRINTER, 
PHILADELPHIA. 


•UNIVERSITY  )) 


PREFACE  TO  FIFTH  EDITION, 


BEING  confronted  with  the  pleasing  necessity  of  writ- 
ing a  preface  for  a  new  edition  of  this  book,  I  have 
just  read  that  which  I  wrote  for  the  preceding  edition. 
I  find  that  the  latter  expresses  about  what  I  have  in 
my  mind,  and  it  is  here  repeated  with  but  little  change. 

In  preparing  this  new  edition,  I  have  again  been 
tempted  to  change  the  book  fundamentally,  and  to  give 
it  a  character  more  in  keeping  with  the  recent  tenden- 
cies in  the  field  of  Physical  or  General  Chemistry. 
But,  taking  everything  into  consideration,  I  have  con- 
cluded to  resist  the  temptation,  and  remain  true  to  the 
original  title  and  character  of  the  book.  Accordingly, 
it  is  essentially  what  it  has  been — a  brief  treatise  on 
those  facts  and  speculations  that  have  to  deal  especially 
with  the  problem  of  the  constitution  of  chemical  com- 
pounds. My  object  has  been  and  is  to  help  students  to 
get  clear  ideas  in  regard  to  the  foundations  of  chemistry. 
That  the  treatment  has  been  regarded  with  some  favor 
is  shown  by  the  fact  that  five  editions  of  the  book  have 
been  called  for  in  a  comparatively  short  time;  and,  fur- 

(iii) 

83070 


iv  PREFACE  TO  FIFTH  EDITION. 

ther,  by  the  fact  that  it  has  been  translated  into  German 
and  Italian.  I  believe  that  all  changes  required  by  the 
advance  of  the  science  have  been  made,  and  that  this 
edition  will  be  found  abreast  of  the  times. 

ISA  KEMSEN. 

BALTIMORE,  December,  1896. 


CONTENTS. 


CHAPTER   I. 

PAGE 

INTRODUCTION 13 

CHAPTEE   II. 

COMBINING-NUMBERS— ATOMIC  WEIGHTS — ATOMIC    THEORY         1 9 

Law  of  definite  proportions — Dalton's  investigations — 
Law  of  multiple  proportions— Atomic  theory — Determina- 
tion of  atomic  weights — Method  for  the  determination  of 
atomic  weights  dependent  upon  analysis — Equivalents — 
Determination  by  Berzelius — The  principle  of  substitution 
employed  in  the  determination  of  atomic  weights — Con- 
sideration of  chemical  decompositions  for  the  purpose  of 
determining  atomic  weights  —  Elements  —  Compounds  — 
Mechanical  mixtures— Solutions  and  alloys. 

CHAPTER   III. 
EXAMINATION  OF  GASEOUS  ELEMENTS  AND  COMPOUNDS    .      34 

Investigations  of  Gay  Lussac — Avogadro's  views — Deter- 
mination of  molecular  weights — Number  of  atoms  in  the 
molecules  of  elements — Molecules  of  elements  which  contain 
more  or  less  than  two  atoms — Varying  number  of  atoms  in 
the  molecule  of  one  and  the  same  element — Other  proofs  of 
the  fact  that  the  molecules  of  elements  contain  more  than 
one  atom — Molecular  formulas  of  gaseous  compounds — 
Apparent  exceptions. 

CHAPTER   IV. 

A  STUDY  OF  SOLUTIONS 60 

Relation  between  the  vapor-pressure  of  solutions  and  the 
molecular  weights  of  the  dissolved  substances — Relation 
between  the  freezing-points  of  solutions  and  the  molecular 
weights  of  the  dissolved  substances — Relation  between  the 

tv) 


vi  CONTENTS. 


osmotic  pressure  of  solutions  and  the  molecular  weights  of 
the  dissolved  substances — Exceptions  to  the  laws  of  solutions, 

CHAPTEK   V. 

EXAMINATION  OF  SOLID  ELEMENTS  AND  COMPOUNDS         .      G5 

Specific  heat — Kelations  between  specific  heat  and  atomic 
weight — Investigations  of  Dulong  and  Petit — Investigations 
of  Neumann  and  Regnault — Determination  of  atomic  weights 
by  a  study  of  the  specific  heat  of  compounds— Exceptions  to 
the  law  of  Dulong  and  Petit — Isomorphism  as  furnishing  a 
means  for  determining  atomic  weights. 

CHAPTER   VI. 

PROPERTIES  OF  THE  ELEMENTS  AS  FUNCTIONS  OF  THEIR 

ATOMIC  WEIGHTS — THE  PERIODIC  LAW    ...      78 
Natural  groups  of  elements — Mendeleeff's  tables — Lothar 
Meyer's  arrangement  of  the  elements. 

CHAPTER   VII. 

VALENCY 88 

Definition — Name  of  the  new  property — Distinction  be- 
tween valency  and  affinity  Methods  for  determining  the 
valency  of  the  elements — Is  valency  constant  or  variable? — 
Variation  of  valency  toward  one  and  the  same  element — 
Atomic  and  molecular  compounds — Foundation  for  the  dis- 
tinction between  atomic  and  molecular  compounds — Use  of 
the  distinction — Difficulties  met  with — Experiments  show- 
ing that  nitrogen  may  be  both  trivalent  and  quinquivalent — 
The  distinction  between  atomic  and  molecular  compounds 
unnecessary  as  far  as  the  hypothesis  of  valency  is  concerned 
— Saturated  and  unsaturated  compounds — Maximum  val- 
ency and  apparent  valency — Maximum  valency  is  depend- 
ent upon  conditions — Are  all  the  bonds  of  an  element  of 
the  same  order— Self- saturation  of  atoms — Single,  double, 
and  triple  linkage  of  atoms  of  the  same  kind — Relative 
valency — Periodic  variations  in  valency — Classification  of 
the  elements  with  reference  to  their  valency — Application 
of  the  views  concerning  valency  to  the  study  of  cheirical 
compounds. 


CONTENTS.  vii 

CHAPTEK  VIII. 

PAGE 

CONSTITUTION  OR  STRUCTURE  OF  CHEMICAL  COMPOUNDS — 

DEFINITION  OF  CONSTITUTION,  ETC Ill 

Definition,  etc. — Linkage  of  atoms. 

CHAPTER   IX. 

CONSTITUTION  OF  CLASSES  OF  COMPOUNDS  .        .        .        .120 

Acids — Hydrogen  acids — Hydroxyl  acids — Sulphur  acids 
— Nitrogen  acids— Double  halogen  acids — Classification  of 
acids — Bases — Differences  between  acids  and  bases — Com- 
plex bases— Salts — Complex  salts— Anhydrides— Experi- 
mental evidence  of  the  constitution  of  anhydrides — Oxides 
— Analogy  between  salts  and  anhydrides  and  oxides. 

CHAPTER   X. 

CONSTITUTION  OF  CLASSES  OF  COMPOUNDS  OF  CARBON       .    136 

Hydrocarbons — Homologous  series — Alcohol — Classes  of 
alcohols — Primary  alcohols — Secondary  alcohols — Evidence 
in  favor  of  the  general  formula  of  secondary  alcohols —Ter- 
tiary alcohols— Mercaptans — Acids— Methods  for  the  forma- 
tion of  acids  of  carbon— Aldehydes— Acetones — Ethereal 
salts — Ethers— Anhydrides. 

CHAPTEK   XI. 
CONSTITUTION  OF  SUBSTITUTION-PRODUCTS  .        .        .        .161 

Substitution -products  containing  chlorine,  bromine,  or 
iodine  —  Complex  substitution-products  —  Constitution  of 
substituting-groups— Constitution  of  the  group  CN — Consti- 
tution of  the  group  SO3H — Constitution  of  the  group  NO2 — 
Constitution  of  the  group  NO— Constitution  of  the  group 
NH2— Constitution  of  the  group  NH— Constitution  of  the 
groups  N2H3  and  N2H2. 

CHAPTER   XII. 

SPECIAL  STUDY  OF  THE  CONSTITUTION  OF  CHEMICAL  COM- 
POUNDS  175 

Compounds  not  containing  carbon,  or  inorganic  com- 
pounds: Compounds  of  chlorine,  etc.,  with  oxygen,  and 


viii  CONTENTS. 

PAGE 

with  oxygen  and  hydrogen— Compounds  of  sulphur,  etc., 
with  oxygen,  and  with  oxygen  and  hydrogen — Sulphurous 
acid— Sulphuric  acid — Pyrosulphuric  acid — Thiosulphuric 
acid — Dithionic  acid — Trithionic  acid — Tetrathionic  acid — 
Pentathionic  acid— Compounds  of  nitrogen  with  oxygen, 
and  with  oxygen  and  hydrogen — Hydroxylamine — Com- 
pounds of  phosphorus  with  oxygen,  and  with  oxygen  and 
hydrogen — Hypophosphorous  acid  —  Phosphorous  acid — 
Phosphoric  acid  —  Pyrophosphoric  acid  —  Metaphosphoric 
acid — Compounds  of  boron  with  oxygen,  and  with  oxygen 
and  hydrogen — Compounds  of  silicon  with  oxygen,  and  with 
oxygen  and  hydrogen — General  remarks  on  the  relations  of 
ordinary  acids  to  the  so-called  normal  acids — Salts — Am- 
monium salts — Salts  of  copper  and  mercury — Salts  of  iron 
and  chromium — Salts  of  aluminium — Metal  acids — Com- 
pounds of  uranium. 

CHAPTEK   XIII. 
CONSTITUTION  OF  CARBON  COMPOUNDS        ....     199 

Methane  derivatives  (Fatty  compounds) :  Compounds 
derived  from  the  hydrocarbons  CnH2n+2 — Derivatives  of 
ethane — Isomerism — Derivatives  of  propane — Derivatives 
of  butane — Derivatives  of  pentane — Derivatives  of  hexane 
— Derivatives  of  heptane. 

CHAPTER  XIV. 

MONOBASIC  ACIDS,  CWH2«O2,  ETC 211 

Propionic  acid — Butyric  acids — Valeric  acids — Caproic 
acids — Aldehydes — Acetones  or  Ketones— Diacid  alcohols, 
CnH2n+2^2 — Monohydroxy-monobasic  acids,  CHH2nO3 — Hy- 
droxy-propionic  acids — Lactones — Dibasic  acids,  CKH2n-2O4 
— Triacid  alcohols  and  tribasic  acids — Glycerol — More  com- 
plex alcohols  and  acids — Cyanogen  compounds — Mustard 
oils— Derivatives  of  carbonic  acid. 

CHAPTEE   XV. 

UNSATURATED  COMPOUNDS  ALLIED  TO   THE    MARSH-GAS 

DERIVATIVES 228 

Ethylene  and  derivatives — Propylene,  etc. — Alcohols- 
Acids — Acetylene— Compounds  containing  a  smaller  pro- 
portion of  hydrogen. 


r  H 

CONTENTS,  ix 


CHAPTEK   XVI. 

PAGE 

BENZENE  DERIVATIVES.    (AROMATIC  COMPOUNDS)      .        .    237 

Constitution  of  benzene— Substitution-products  of  ben- 
zene— Di-substitution-products — Tri-substitution  -products — 
Peculiar  benzene  derivatives— Phenols — Quinones — Azo- 
and  diazo-compounds— Azoxy-  and  hydrazo-compounds — 
Phenylmethanes — Rosaniline  and  pararosaniline — Phtha- 
leins — Phenylethylene— Furfuran  —  Pyrrol  —  Thiophene — 
Naphthalene — Pyridine  and  quinoline — Anthracene — Re- 
trospect. 

CHAPTER   XVII. 

PHYSICAL    METHODS   FOR   THE   DETERMINATION  OF   THE 

CONSTITUTION  OF  CHEMICAL  COMPOUNDS  .  .  .  273 
General  —  Specific  volume  —  Molecular  refraction  —  Me- 
thods dependent  upon  determinations  of  the  amount  of  heat 
evolved  in  chemical  reactions  or  thermal  methods— Heat  of 
formation — Heat  of  neutralization — Magnetic  rotary  polari- 
zation in  relation  to  chemical  constitution— The  shape  of 
molecules — Stereochemistry. 

CHAPTER   XVIII. 

THE  STUDY  OF  CHEMICAL  AFFINITY 293 

Introduction — The  nature  of  the  problem — Rough  mea- 
surements of  affinities — Disturbing  influences — Attempts  to 
measure  affinity  by  observations  on  the  heat  evolved  in 
chemical  reactions — Resultant  affinity — Heat  of  neutraliza- 
tion— Avidity  of  acids — Mass-action — Measurement  of  co- 
efficients of  affinity  —  Volume-chemical  method  —  Specific 
coefficient  of  affinity — Methods  for  determining  specific  co- 
efficients of  affinity. 

CHAPTER   XIX. 

CONNECTION  BETWEEN  THE  CHEMICAL  CONSTITUTION  AND 

PROPERTIES  OF  COMPOUNDS 306 

General — 1.  Change  of  character  in  certain  parts  of  a 
compound  caused  by  the  introduction  of  some  atom  or 
group— Bases,  alcohols,  acids— Influence  of  acid  groups  like 


CONTENTS. 

NO2 — Change  in  the  chemical  character  of  ammonia — 
Influence  of  the  nitro  group  on  chlorine — Substitution  in 
hydrocarbons— Oxidation-phenomena— 2.  Tendency  on  the 
part  of  certain  compounds  to  break  down  in  certain  ways 
— Anhydrides — Lactones — Lactams  and  Lactims — Other  an- 
hydro-compounds — Elimination  of  carbon  dioxide — Conclu- 
sions warranted  by  the  facts  just  presented — Breaking  down 
of  unsaturated  compounds — 3.  Influence  exerted  by  certain 
atoms  or  groups  in  a  compound  on  the  constitution  of  the 
products  formed  by  further  acts  of  substitution— Substitu- 
tion in  symmetrical  compounds — Influence  of  acid  or  nega- 
tive groups  on  groups  of  the  same  kind— Influence  of  basic 
or  positive  groups — Regularities  in  the  addition  of  negative 
or  acid  atoms  to  unsaturated  compounds— 4.  Eelative  ease 
with  which  isomeric  compounds  enter  into  action — Decom- 
position of  halogen  derivatives. 


PRINCIPLES 


OF 


THEORETICAL  CHEMISTRY 


CHAPTER  I. 


INTRODUCTION. 

THE  science  of  chemistry  has  to  deal  with  everything 
connected  with  the  deepest-seated  changes  in  composition 
which  the  different  forms  of  matter  undergo.  The  first 
observations,  and  for  a  long  time  the  only  observations, 
made  on  chemical  changes  were  qualitative.  The  fact  that 
a  substance  A  when  brought  in  contact  with  a  substance 
B  gives  the  substance  C  or  the  substances  C  and  D  was 
noted  ;  but  very  little,  if  anything,  more  was  learned  re- 
garding the  change.  During  this  qualitative  period  a 
great  many  facts  were  discovered,  many  new  substances 
were  brought  to  light,  and  general  methods  of  preparation 
for  certain  classes  of  compounds  were  devised. 

Toward  the  end  of  the  last  century  the  researches  of 
Lavoisier  made  it  clear  to  chemists  that  in  studying  chem- 
ical changes  it  is  necessary  to  take  into  account  not  only 
the  nature  of  the  substances  which  are  involved  in  reac- 
tions, but  also  the  quantities  of  these  substances.  Thus 
the  quantitative  period  of  chemistry  was  begun.  Since 
then  chemists  have  paid  special  attention  to  the  weights  of 
the  substances  with  which  they  worked,  and  it  is  largely 
due  to  this  that  the  science  has  advanced  so  rapidly  during 
the  last  hundred  years.  It  was  soon  shown  that  certain 
laws  underlie  all  chemical  changes,  and  the  recognition  of 
these  laws  proved  of  the  greatest  assistance  in  the  further 
study  of  the  changes. 

2  (13) 


14      PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

The  first  law  of  general  application  that  was  recognized 
was  the  law  of  the  indestructibility  of  matter.  It  was  found 
as  the  result  of  many  observations  that  the  sum  of  the 
masses  of  the  products  of  a  chemical  change  is  always 
exactly  equal  to  the  sum  of  the  masses  of  the  substances 
which  act  upon  one  another.  Nothing  is  gained  or  lost  in 
the  operation.  This  law,  like  every  other  law  of  nature, 
is  simply  a  statement  of  what  has  been  found  to  be  true  in 
every  case  that  has  been  studied,  and  its  statement  implies 
that  a  large  number  of  cases  have  been  studied.  Since  its 
truth  was  first  recognized,  an  almost  infinite  number  of 
observations  have  confirmed  it  in  the  most  striking  way. 
As  regards  the  question  whether  it  is  possible  to  conceive 
of  a  state  of  things  in  which  the  law  of  the  indestructibility 
of  matter  would  not  hold  good  we  have  nothing  to  do.  It 
may  be  said,  however,  that  the  law  could  not  have  been 
discovered  without  the  aid  of  many  quantitative  chemical 
experiments.  It  is  an  expression  of  facts  established.  It 
is  not  an  axiom. 

The  next  law  discovered  was  the  law  of  definite  propor- 
tions; and  almost  at  the  same  time  came  the  law  of  multiple 
proportions.  These  two  laws  are  the  expressions  of  the 
facts  learned  in  studying  the  proportions  in  which  the 
elements  combine  with  one  another. 

The  discovery  of  a  law  naturally  leads  to  a  desire  to 
explain  it.  When  we  have  recognized  that  elements  com- 
bine according  to  the  laws  of  definite  and  multiple  propor- 
tions, we  next  ask,  Why  do  they  combine  in  this  way  ? 
An  attempt  to  answer  this  question  leads  to  some  sugges- 
tion which  is  known  as  an  hypothesis.  If,  after  it  has  been 
thoroughly  tested  by  further  extensive  study  of  the  facts, 
the  hypothesis  is  found  to  be  in  harmony  with  all  the  facts, 
and  capable  of  explaining  them,  it  is  then  called  a  theory. 
The  atomic  hypothesis  was  put  forward  to  explain  the  laws 
of  definite  and  multiple  proportions,  and  this  hypothesis 
has  proved  of  great  value  in  enabling  chemists  to  deal 
with  the  facts  of  chemistry.  It  has  long  since  been  ac- 
cepted as  a  satisfactory  theory. 

Besides  the  laws  mentioned  several  others  bearing  upon 
the  proportions  by  weight  or  by  volume  in  which  sub- 
stances act  upon  one  another  have  been  discovered,  and  all 
have  been  found  to  be  explicable  by  the  aid  of  the  atomic 
hypothesis.  Among  them  may  be  mentioned  the  law  of 


INTRODUCTION.  15 

combination  by  volume,  the  law  of  specific  heats,  and, 
most  comprehensive  of  all,  the  periodic  law,  expressing 
the  fact  that  the  properties  of  the  elements  are  functions 
of  their  atomic  weights,  that  the  properties  of  any  element 
are  determined  by  its  atomic  weight. 

Again,  laws  governing  the  complexity  of  chemical  com- 
pounds have  been  discovered.  It  has  been  found  that 
there  is  a  law  that  limits  the  number  of  atoms  of  one 
element  that  can  combine  with  one  atom  of  another.  A 
careful  consideration  of  the  facts  upon  which  this  law  is 
based  has  led  to  certain  conceptions  in  regard  to  the  struc- 
ture or  constitution  of  all  chemical  compounds ;  and  it  has 
been  found  that,  by  a  study  of  the  transformations  and  of 
the  methods  of  formation  of  compounds,  definite  conclu- 
sions regarding  their  structure  can  be  reached.  Similarity 
of  structure  is  found  to  be  the  cause  of  similarity  of  prop- 
erties. By  studies  on  the  constitution  of  chemical  com- 
pounds a  rational  classification  of  chemical  reactions  was 
made  possible,  and  the  study  of  these  reactions  was  very 
much  simplified. 

Up  to  the  present  most  chemical  investigations  have 
had  for  their  object  the  determination  of  the  constitution 
of  compounds  and  much  still  remains  to  be  done.  Indeed, 
but  little  more  than  a  beginning  has  been  made  in  this 
direction.  Much  more  exact  studies  are  called  for  in  many 
cases,  and  most  of  the  laws  governing  the  connection  be- 
tween constitution  and  chemical  reactions  are  still  undis- 
covered. We  are  beginning  the  quantitative  period  in  the 
study  of  constitution.  The  results  thus  far  achieved  are 
of  such  importance  that  we  may  confidently  look  forward 
to  still  further  great  advances — probably  greater  than 
those  already  recorded. 

The  main  object  in  view  in  this  book  is  to  point  out  as 
clearly  as  possible  the  reasons  for  accepting  the  prevailing 
views  in  regard  to  constitution,  to  show  that  these  views 
are  not  merely  products  of  the  imagination,  but  that  they 
are  the  legitimate  results  of  a  profound  and  comprehensive 
study  of  chemical  phenomena,  and  that  they  are  the  sim- 
plest views  possible,  if  we  accept  as  the  basis  of  speculation 
the  atomic  theory.  Whatever  fate  may  await  the  prevail- 
ing theories,  it  is  certain  that  the  facts  expressed  in  our 
present  chemical  formulas  must  find  expression  in  their 
successors.  The  formulas  of  the  future  will  express  all 


16       PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

the  facts  expressed  by  the  formulas  of  the  present,  besides 
others  yet  to  be  discovered. 

Just  as  chemical  studies  have  shown  that  similarity  in 
chemical  conduct  implies  similarity  in  constitution,  so 
studies  of  the  physical  properties  of  compounds  have  shown 
that  similiarity  in  constitution  is  connected  with  similarity 
in  certain  physical  properties,  and  the  further  investigation 
has  been  pushed  along  this  line,  the  closer  has  this  connec- 
tion appeared  to  be.  It  is  in  this  direction  that  we  must 
look  for  more  definite  views  in  regard  to  constitution  than 
we  have  at  present.  Chemists  are  now  engaged  very  largely 
in  classifying  chemical  compounds  according  to  their  reac- 
tions without  having  any  clear  conception  or  any  hypothe- 
sis in  regard  to  the  physical  structure  of  the  simplest  chemi- 
cal compounds.  And  yet,  though  we  have  no  hypothesis 
in  regard  to  the  physical  structure  of  chemical  compounds, 
it  requires  no  argument  to  convince  us  that  every  chemical 
compound  has  a  definite  physical  structure ;  and  the  dis- 
covery of  the  structure  of  any  one  compound  would  give 
us  an  insight  into  all  chemical  compounds.  If  now,  as 
investigation  is  pushed  further  and  further,  it  can  be  shown 
that  certain  physical  properties  always  accompany  a  certain 
chemical  constitution,  we  may  finally  be  able  to  interpret 
our  views  regarding  structure  in  physical  terms.  A  brief 
chapter  of  this  book  is  devoted  to  a  presentation  of  those 
methods  which  have  been  of  most  service  in  showing  a 
connection  between  physical  properties  and  chemical  con- 
stitution. 

However  important  the  study  of  the  constitution  of 
chemical  compounds  may  be,  and  no  one  doubts  its  great 
importance,  there  is  much  to  be  learned  regarding  chemi- 
cal phenomena,  besides  the  constitution  of  the  substances 
which  take  part  in  a  reaction.  We  might  know  the  con- 
stitution of  every  compound, — important  questions  would 
still  remain  to  be  answered.  The  one  great  question 
is,  What  is  the  cause  of  chemical  action  ?  We  may  in 
the  same  way  be  thoroughly  acquainted  with  the  phe- 
nomena of  the  motion  of  the  heavenly  bodies  and  of  fall- 
ing bodies  without  having  any  conception  in  regard  to 
the  cause  of  these  phenomena.  We  may  be  familiar  with 
the  phenomena  of  light  and  not  know  the  cause  of  these 
phenomena;  and  so  with  the  phenomena  of  heat  and 
sound  and  electricity  and  magnetism.  As  regards  the 


INTRODUCTION.  17 

cause  of  the  phenomena  of  the  motion  of  the  heavenly 
bodies  and  of  falling  bodies  we  have  no  conception  at  the 
present  day.  It  is  true,  we  say  that  these  phenomena  are 
caused  by  the  attraction  of  gravitation,  and  certain  laws 
governing  this  attraction  have  been  determined,  and  if  we 
assume  that  all  bodies  are  pulled  together  by  a  force  which 
is  proportional  to  the  masses  of  the  bodies,  and  inversely 
proportional  to  the  squares  of  the  distances,  we  have  an 
explanation  of  the  phenomena;  but,  after  all,  we  do  not 
know  what  pulls  these  bodies  together.  The  phenomena 
of  heat  and  of  sound  have  found  a  much  more  satisfactory 
explanation  in  the  suggestion  that  they  are  due  to  the 
rapid  motion  of  the  bodies  themselves,  or  of  the  small 
particles  of  which  the  bodies  are  believed  to  be  composed. 
In  the  mechanical  theory  of  heat  we  have  probably  the 
most  satisfactory  theory  put  forward  to  account  for  a  class 
of  natural  phenomena.  Not  many  years  ago  heat  phe- 
nomena were  explained  by  assuming  that  there  was  a  sub- 
stance, caloric,  which  could  be  put  into  substances  and 
taken  out  of  them. 

As  regards  the  question,  then,  What  is  the  cause  of 
chemical  action?  no  satisfactory  answer  can  be  given. 
Indeed,  in  most  of  the  investigations  which  have  thus 
far  been  carried  on  no  attempt  has  been  made  to  answer 
it.  These  investigations  have  had  to  do  almost  entirely 
with  questions  of  composition  and  constitution.  What 
are  the  substances  brought  together,  and  what  are  the 
substances  formed  ?  have  been  the  main  questions  asked  ; 
and  but  little  attention  has  been  given  to  the  action  itself. 
The  changes  brought  about  have  been  ascribed  to  the 
action  of  a  special  force,  called  chemical  affinity;  a  force 
which  was  considered  to  have  its  seat  in  the  atoms,  and 
to  act  between  atoms  in  somewhat  the  same  way  that  the 
attraction  of  gravitation  is  considered  to  act  between 
masses.  From  time  to  time  attempts  have  been  made  to 
learn  more  in  regard  to  the  nature  of  this  force.  At  one 
time  chemical  attraction  was  regarded  as  essentially  iden- 
tical with  electrical  attraction ;  again,  it  was  regarded  as 
identical  with  the  attraction  of  gravitation.  Both  of  these 
views  have  been  shown  to  be  untenable.  Of  late  this  sub- 
ject has  received  considerable  attention,  and  in  the  work 
that  has  been  done  there  is  much  promise.  Studies  have 
been  made  on  the  velocity  of  various  chemical  reactions ; 


18       PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

on  the  influence  of  mass  on  chemical  action ;  on  chemical 
equilibrium ;  on  the  influence  of  temperature  on  the  course 
of  chemical  reactions,  and  various  other  phenomena,  a 
detailed  knowledge  of  which  is  necessary,  in  order  that 
a  satisfactory  theory  of  chemical  affinity  may  be  framed. 
A  brief  chapter  will  be  devoted  to  an  account  of  the  chief 
results  obtained  in  investigations  on  chemical  affinity. 
In  this  chapter  there  will  be  some  reference  to  the  con- 
nection between  heat  phenomena  and  chemical  action,  but 
no  attempt  will  be  made  to  treat  the  subject  of  thermo- 
chemistry in  anything  but  the  broadest  outlines,  notwith- 
standing the  fact  that  work  in  this  field  has  been  carried 
on  with  great  industry  for  more  than  twenty  years,  and  many 
results  have  been  attained  which  are  of  importance  in  the 
study  of  chemical  phenomena.  So  far  as  these  results 
have  a  direct  bearing  upon  the  subject  of  chemical  consti- 
tution, or  upon  the  nature  of  chemical  affinity,  they  will 
receive  attention. 

From  what  has  been  said,  it  will  be  clear  that  we  have 
as  yet  no  theory  of  chemical  affinity,  as  we  have  a  theory 
of  heat.  We  are  just  at  the  beginning  of  the  study  of  the 
laws  of  chemical  affinity,  a  knowledge  of  which  must  pre- 
cede the  formation  of  any  clear  conception  of  the  force. 
The  time  may  come  when  we  shall  have  a  theory  of  chemi- 
cal affinity,  in  the  light  of  which  all  chemical  phenomena 
will  appear  much  simpler  than  they  do  now,  just  as  all  heat 
phenomena  find  a  ready  explanation  in  the  mechanical 
theory  of  heat.  This  theory  of  affinity  will  probably  be 
an  extension  of  our  present  atomic  theory — it  will  have  to 
do  with  the  action  of  atoms,  and  not  merely  with  their 
existence  in  a  condition  of  equilibrium  ;  in  other  words,  it 
will  deal  not  alone  with  the  statical  side  of  chemical  phe- 
nomena, but  with  the  dynamical. 


COMBINING  NUMBERS,  ETC.  19 


CHAPTER    II. 

COMBINING   NUMBERS — ATOMIC   WEIGHTS — ATOMIC 
THEORY. 

Law  of  Definite  Proportions. — The  first  fundamental  law 
governing  all  transformations  of  matter  is,  as  has  been 
stated,  the  law  of  the  indestructibility  of  matter.  Next  in 
order  of  discovery  came  the  law  of  definite  proportions. 
According  to  this  : — 

Every  chemical  compound  always  contains  the  same 
constituents  in  the  same  proportion  by  weight. 

This  law,  though  perhaps  tacitly  acknowledged  by  most 
chemists,  was  not  fully  established  until  the  beginning  of 
the  present  century.  In  1803  a  strong  effort  was  made  by 
Berthollet,  in  his  work  entitled  Statique  Chimique,  to  show 
that  the  law  is  not  true,  but  the  opposition  called  forth  by 
this  work,  particularly  from  Proust,  led  to  more  and  more 
careful  examinations  of  chemical  compounds,  and  thus  to 
the  firm  establishment  of  the  law.  Proust  also  showed  that 
two  elements  can  combine  with  each  other  in  more  than 
one  proportion,  and  that  for  each  compound  thus  formed 
the  proportions  of  the  constituents  are  fixed. 

Dalton's  Investigations,  Law  of  Multiple  Proportions. — 
In  the  year  1804  Dalton's  investigations  enabled  him  to 
take  another  advance  step.  Another  general  law  govern- 
ing chemical  action  was  discovered  and  propounded.  This 
is  the  law  of  multiple  proportions.  As  this  is  of  fundamental 
importance,  it  will  be  well  to  follow,  somewhat  in  detail, 
Dalton's  reasoning.  Many  substances  had  been  analyzed 
before  his  time,  and  the  percentages  of  the  constituents  had 
been  determined  with  a  fair  degree  of  accuracy.  He 
examined,  first,  two  gases,  both  of  which  consist  of  carbon 
and  hydrogen,  viz.,  olefiant  gas  and  marsh-gas.  He  ana- 
lyzed them  both,  and  determined  the  percentages  of  the 


20      PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

constituents  contained  in  them.     These  percentages  are  as 
follows : — * 

Olefiant  gas,  85.7  per  cent,  carbon,  and  14.3  per  cent  hydrogen. 
Marsh-gas,     75.0        "  25.0         " 

On  comparing  these  numbers  he  found  that  the  ratio  of 
carbon  to  hydrogen  in  olefiant  gas  is  asjgjoj^  whereas, 
in  marsh-gas  it  is  as  3  to  1,  or  6  to  2.  ^ThVweighTof  hy- 
drogen, combined  with  a  given  weight  of  carbon,  is  exactly 
twice  as  great  in  the  one  case  as  in  the  other. 

For  the  two  oxides  of  carbon,  further,  the  following 
figures  were  obtained : — 

Carbon  monoxide,  42.86  p.  c.  carbon,  and  57.14  p.  c.  oxygen. 
Carbon  dioxide,       27.27  72.73 

But  42.86  :  57.14jjJ5jj^and  27.27  :  72.73  :  :  6  :  16. 

The  weight  of  oxygen,  combined  with  a  given  weight  of 
carbon  in  carbon  dioxide,  is  exactly  twice  as  great  as  the 
weight  of  oxygen  combined  with  the  same  weight  of  car- 
bon in  carbon  monoxide.  He  saw,  again,  that,  in  olefiant 
gas,  one  part  by  weight  of  hydrogen  combines  with  six 
parts  by  weight  of  carbon,  and  that,  in  carbon  monoxide, 
eight  parts  by  weight  of  oxygen  combine  also  with  six 
parts  by  weight  of  carbon.  Water  was  now  examined. 
It  contains  88,89  per  cent,  oxygen  and  11.11  per  cent, 
hydrogen,  and  these  numbers  are  to  each  other  as  8  to  1. 
The  numbers  which,  in  the  first  place,  represent  the  com- 
bining proportions  of  oxygen  and  hydrogen,  respectively, 
with  carbon,  are  also  found  to  represent,  in  the  second 
place,  the  combining  proportions  of  oxygen  and  hydrogen 
with  each  other.  Subsequent  examination  of  other  com- 
pounds led  to  similar  results,  and  thus  Dalton  had  discov- 
ered the  law  of  multiple  proportions.f  This  may  be  stated 
as  follows : — 

*  Instead  of  the  figures  actually  found  by  Dalton,  the  corrected 
figures  are  given,  for  the  sake  of  simplicity. 

f  According  to  recent  investigations,  it  appears  that  the  above 
statement  is  not  strictly  in  accordance  with  the  facts.  Dalton  seems 
to  have  been  led  to  the  discovery  of  the  law  of  multiple  propor- 
tions through  speculations  on  the  constitution  of  matter.  He  never 
gave  to  the  world  an  account  of  the  way  in  which  he  reached  his 
conclusions ;  and  it  is  to  Thomson,  a  contemporary,  that  we  owe  the 
statement  above  given. 


COMBINING  N UMBEKS,'UTC.  21 

If  two  substances,  A  and  B,  form  several  compounds 
with  each  other,  and  we  consider  any  fixed  mass  of  A, 
then  the  different  masses  of  B,  which  combine  with  this 
fixed  mass  of  A,  bear  a  simple  relation  to  each  other. 
This  law  has  been  fully  confirmed  by  all  investigations 
carried  on  since  the  time  of  Dalton. 

Atomic  Theory. — But  Dalton  did  not  stop  with  the  dis- 
covery of  the  law  of  multiple  proportions ;  he  sought  for 
its  explanation.  He  was  thus  led  to  propose  the  atomic 
hypothesis,  as  affording  the  simplest  explanation  of  the  facts 
observed. 

The  question  as  to  the  ultimate  constitution  of  matter 
had  frequently  and  from  the  earliest  dates  been  discussed. 
Two  views  were  held  at  different  periods,  and  by  different 
thinkers.  According  to  one  of  these,  matter  was  supposed 
to  be  infinitely  divisible ;  according  to  the  other,  it  was 
supposed  that  there  is  a  limit  to  the  divisibility,  and  that 
this  limit  is  reached  when  the  division  has  been  carried  to 
certain  small  particles  called  atoms.  After  the  discovery 
of  the  law  of  multiple  proportions,  however,  the  atomic 
theory  acquired  a  more  definite  form,  as  the  existence  of 
atoms  was  supposed  to  have  a  direct  connection  with 
chemical  combinations.  The  results  of  Dalton's  investi- 
gations are  not  fully  stated  in  the  law  of  multiple  propor- 
tions as  above  given ;  another  fact  was  made  clear,  which 
is  also  of  importance.  The  complete  results  may  be  stated 
as  follows :  For  each  element  a  particular  number  may  be 
selected,  and  this  number,  or  a  simple  multiple  of  it,  repre- 
sents the  proportion  by  weight  in  which  this  element  combines 
with  other  elements.  This  is  a  fact,  which  involves  no 
hypothesis  as  to  the  nature  of  matter.  The  "combining 
numbers"  of  the  elements  may  be  used  without  reference 
to,  or  thought  of,  the  existence  of  atoms.  But  the  question 
naturally  suggests  itself:  Why  do  elements  combine  accord- 
ing to  the  laws  of  definite  and  multiple  proportions?  No 
absolute,  final  answer  can  be  given  to  this  question,  but  we 
can  imagine  a  cause,  or,  as  it  is  commonly  expressed,  we  can 
propose  an  hypothesis.  This  Dalton  did.  He  supposed 
that  chemical  action  takes  place  between  atoms,  i.  e.,  be- 
tween particles  that  are  indivisible  and  have  definite 
weights.  If  chemical  combination  takes  places  between 
one  atom  of  one  substance  and  one  atom  of  another  sub- 

2* 


22       PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

stance,  or  between  a  simple  number  of  atoms  of  one  sub- 
stance and  a  simple  number  of  atoms  of  another,  and  these 
atoms  have  definite  weights,  then,  indeed,  the  explanation 
of  the  laws  of  definite  and  multiple  proportions  is  given. 

Thus  the  idea  of  atoms  became  a  much  more  tangible 
one  than  it  had  been  up  to  that  time.  Not  only  were 
atoms  supposed  to  have  definite  weights,  but  a  method 
was  suggested  by  means  of  which  their  relative  weights 
could  be  determined.  The  number  assigned  to  an  element, 
representing  its  combining  weight,  also  represents  the  rela- 
tive weight  of  its  atoms.  The  fact  that  the  combining 
weight  of  an  element  was  in  some  cases  represented  by 
a  multiple  of  the  simplest  number  was  satisfactorily  ac- 
counted for  by  supposing  that  in  these  cases  more  than  one 
atom  of  the  element  combined  with  one  of  another  element. 

Determination  of  Atomic  Weights. —  The  determination 
of  combining  numbers  or  atomic  weights,  as  the  figures 
were  indiscriminately  called  by  some,  became  now  the 
chief,  immediate  problem  of  the  science  of  chemistry. 
Dalton's  atomic  hypothesis  was  accepted  by  many,  though 
not  by  aUL  The  laws  governing  chemical  combinations 
could  not  be  doubted,  but  the  explanation  could  be  and  was. 
Nevertheless,  the  importance  of  determining  for  each  ele- 
ment the  characteristic  number,  call  it  atomic  weight  or 
combining  weight,  or  combining  number,  was  generally 
acknowledged ;  and  consequently  particular  attention  was 
given  to  this  field  of  research  during  the  period  directly 
following  the  time  of  Dalton's  publication.  Let  us  see 
how  thoroughly  the  desired  object  could  be  accomplished 
by  the  aid  of  the  principles  laid  down  by  Dalton. 

At  the  time  of  which  we  are  speaking  the  methods  of 
chemical  analysis  were  far  from  perfect,  and  hence  most  of 
the  determinations  then  made  required  subsequent  correc- 
tions which  were  gradually  made  as  analytical  methods  were 
improved.  This  fact  has,  however,  nothing  to  do  with  the 
subject  under  consideration.  The  principle  alone  is  in- 
volved. The  question  to  be  answered  is :  Can  the  relative 
weights  of  the  atoms  be  determined  by  the  method  used 
by  Dalton  ?  To  decide  this  question  we  must  first  examine 
the  method  more  carefully.  In  the  following  discussion 
the  correct  numbers,  as  given  by  later  analyses,  are  em- 
ployed, instead  of  those  originally  found.  This  does  not 


COMBINING  NUMBERS,  ETC.  23 

interfere  with  the  principle,  and  does  simplify  the  subject 
otherwise. 

Method  for  the  Determination  of  Atomic  Weights  de- 
pendent upon  Analysis. — As  the  standard  the  combining 
weight  of  hydrogen  was  first  selected,  and  this  made  1. 
Hydrogen  combines  with  oxygen  in  the  proportion  of 
1:8;  and  as  water  was  the  only  compound  of  hydrogen 
and  oxygen  known,  the  conclusion  was  drawn  that  the 
two  elements  were  united  in  the  simplest  way ;  that  is  to 
say,  one  atom  of  the  one  element  to  one  of  the  other,  and 
hence  the  atomic  weight  of  oxygen  is  8.  Further,  nitrogen 
is  combined  with  hydrogen  in  ammonia  in  the  proportion  of 
one  part  by  weight  of  hydrogen  to  4|  parts  by  weight  of 
nitrogen.  Ammonia  was  the  only  compound  of  nitrogen 
and  hydrogen  known  ;  and  the  same  reasoning  as  that  above 
employed  led  to  the  conclusion  that  the  atomic  weight  of 
nitrogen  is  4f .  Considering  for  a  moment  these  two  sim- 
ple cases,  we  see  that  the  numbers  thus  found,  as  repre- 
senting the  relative  weights  of  the  atoms  of  oxygen  and 
nitrogen,  are  founded  partially  upon  hypothesis.  There  is 
nothing  to  decide  as  to  the  number  of  atoms  of  hydrogen 
and  oxygen  contained  in  water,  nor  of  nitrogen  and  hydro- 
gen in  ammonia,  and,  of  course,  as  long  as  this  number  is 
unknown,  it  is  impossible  to  draw  a  positive  conclusion 
with  reference  to  the  atomic  weights  of  nitrogen  and 
oxygen.  A  conclusion  to  be  of  value  must  be  based 
upon  a  thorough  knowledge  of  the  compounds  of  the  par- 
ticular element  under  consideration.  Such  a  number  must 
finally  be  selected  as  is  most  in  accordance  with  the  facts. 
The  selection  must  remain  more  or  less  arbitrary,  as  can 
be  shown  more  clearly. 

Take  again  the  case  of  oxygen.  A  second  compound 
of  hydrogen  and  oxygen  is  now  known.  This  contains 
the  elements  in  the  proportion  1:16.  At  first  sight  the 
explanation  of  this  appears  simple  enough.  In  this  second 
compound  there  are  two  atoms  of  oxygen  combined  with 
one  of  hydrogen,  and  thus  the  proportion  is  satisfied. 
But  may  we  not  with  equal  right  hold  that  in  water 
there  are  two  atoms  of  hydrogen  combined  with  one  of 
oxygen?  This  would  give  us  for  oxygen  the  atomic 
weight  16,  and,  in  the  second  compound,  there  would  be 
one  atom  of  each  of  the  elements. 


24       PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

Further,  if  the  attempt  is  made  to  determine  the  atomic 
weight  of  carbon  by  Dalton's  method,  difficulties  fully  as 
great  are  encountered,  and  the  final  selection  among  many 
numbers  must  be  more  or  less  arbitrary.  Taking  olefiant 
gas,  we  have  hydrogen  combined  with  carbon  in  the  pro- 
portion 1:6;  in  marsh- gas  the  proportion  is  1:3  or 
2:6.  If  we  suppose  that  in  olefiant  gas  the  elements 
are  combined  atom  to  atom,  then  the  atomic  weight  of 
carbon  is  6,  and  consequently  in  marsh-gas  there  must  be 
two  atoms  of  hydrogen  combined  with  one  atom  of  car- 
bon. But  here  again  it  can  just  as  well  be  assumed  that 
in  marsh-gas  there  is  the  simplest  kind  of  combination, 
and  this  would  give  3  for  the  atomic  weight  of  carbon. 
Then  in  olefiant  gas  two  atoms  of  carbon  would  be  com- 
bined with  one  atom  of  hydrogen. 

Finally,  let  us  take  the  oxygen  compounds  of  carbon. 
In  carbon  monoxide,  carbon  is  combined  with  oxygen  in 
the  proportion  6  :  8  or  3  :  4,  whereas  in  carbon  dioxide 
the  corresponding  proportion  is  6  :  16  or  3  :  8.  Now  let 
us  suppose  the  atomic  weight  of  oxygen  to  be  8.  Then, 
if  carbon  monoxide  is  the  simpler  of  the  two  compounds, 
the  atomic  weight  of  carbon  is  6 ;  and  in  carbon  dioxide 
there  are  two  atoms  of  oxygen  combined  with  each  atom 
of  carbon.  Here,  again,  it  is  evident  that  carbon  dioxide 
may  just  as  well  be  assumed  to  be  the  simpler  compound, 
in  which  case  the  atomic  weight  of  carbon  would  be  3, 
and  in  carbon  monoxide  there  would  be  two  atoms  of 
carbon  combined  with  one  atom  of  oxygen.  Between  these 
different  possibilities  it  is  impossible  to  decide  by  chemical 
analysis.  Similar  instances  might  be  multiplied  indefi- 
nitely ;  the  inadequacy  of  the  method  could  be  made  more 
strikingly  clear  by  examples  of  a  more  complicated  kind, 
but  the  cases  mentioned  are  sufficient  for  the  purpose ;  we 
must  have  other  methods  for  the  determination  of  atomic 
weights  before  we  can  get  numbers  that  are  not  more  or 
less  arbitrary. 

Equivalents. — This  necessity  was  first  clearly  recognized 
by  Wollaston  in  1814.  As  no  satisfactory  method  for  the 
determination  of  atomic  weights  suggested  itself,  he  pro- 
posed to  abandon  the  idea  of  atomic  weights  entirely,  and 
to  substitute  for  it  that  of  the  equivalent,  thus,  as  he  sup- 
posed, getting  rid  of  all  hypotheses  and  obtaining  numbers 
that  are  simple  expressions  of  facts.  The  equivalent  of 


COMBINING  NUMBERS,  ETC.  25 

an  element  was  to  him  that  quantity  of  the  element  that 
possessed  the  same  chemical  value  as  a  given  quantity  of 
another  element,  that  quantity  of  an  element  that  could 
take  the  place  of  a  given  quantity  of  another  element. 
According  to  the  conditions  of  this  definition,  it  is  plain 
that,  in  order  to  know  what  portions  of  two  elements  are 
equivalent,  we  must  be  able  to  compare  the  two.  Hence, 
primarily,  only  of  such  elements  as  can  be  compared  with 
each  other,  of  such  as  possess  a  certain  degree  of  similarity, 
can  the  equivalent  quantities  be  determined.  As  this  direct 
comparison  is  not  always,  nor,  indeed,  in  the  majority  of 
cases,  possible,  recourse  must  be  had  to  indirect  comparison. 

To  illustrate  this  let  us  take  an  example:  Hydrogen 
and  chlorine  combine  with  each  other  in  the  proportion  of 
1  part  by  weight  of  hydrogen  to  35.4  parts  by  weight  of 
chlorine,  and  from  this  fact  the  conclusion  is  drawn  that 
35.4  parts  of  chlorine  are  equivalent  to  1  part  of  hydro- 
gen. In  the  same  way  it  is  found  that  8  parts  of  oxygen, 
80  of  bromine,  16  of  sulphur,  are  all  equivalent  to  1  part 
of  hydrogen.  KnowiDg  that  35.4  is  the  equivalent  of 
chlorine,  the  quantities  of  sodium  and  silver  that  are  re- 
spectively equivalent  to  this  quantity  of  chlorine  are  now 
determined.  For  sodium  23  is  found,  and  for  silver  107.7. 
These  quantities  of  silver  and  sodium  are  further  found  to 
be  equivalent  to  8  parts  of  oxygen,  79.8  parts  of  bromine, 
and  16  parts  of  sulphur,  and  hence  the  conclusion  is  drawn 
that  they  are  also  equivalent  to  1  part  of  hydrogen.  Thus 
the  equivalents  of  sodium  and  silver  have  been  determined 
by  the  method  of  indirect  comparison.  In  most  simple  cases 
this  method  of  procedure  is  justifiable,  but  it  must  be  dis- 
tinctly borne  in  mind  that  such  numbers  as  are  determined 
by  indirect  comparison  with  the  standard,  whatever  this 
may  be,  are  not  in  the  strictest  sense  expressions  of  facts ; 
the  last  step  in  the  determinations,  however  justified  we 
may  be  in  taking  it,  involves  speculation. 

If  the  difficulty  thus  referred  to  were  the  only  one  met 
with  in  the  determination  of  equivalent  numbers,  such 
determinations  would  have  nearly  the  full  value  claimed 
for  them  by  Wollaston.  This,  however,  is  not  the  case. 
In  dealing  with  any  but  the  simplest  compounds  we  are 
left  in  fully  as  much  doubt  in  regard  to  the  equivalent 
numbers  as  we  are  in  regard  to  atomic  weights.  If  it  is 
required  to  determine  the  quantity  of  carbon  that  is  equiva- 


26       PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

lent  to  1  part  of  hydrogen,  the  compounds  of  the  two  ele- 
ments must  be  examined.  But  there  are  a  great  many 
compounds  of  these  two  elements.  Taking  the  two,  olefiant 
gas  and  marsh-gas,  it  is  found  that  in  the  former  (see  ante, 
p.  24)  1  part  of  hydrogen  is  combined  with  (equivalent  to) 
6  parts  of  carbon ;  whereas,  in  the  latter,  1  part  of  hydrogen 
is  combined  with  (equivalent  to)  3  parts  of  carbon.  What 
shall  here  decide  which  is  the  correct  number?  It  is 
evident  from  such  instances  as  this  that  the  idea  of  the 
equivalent  is  fully  as  uncertain  as  that  of  the  atom  was  at 
the  time  of  Wollaston.  That  an  element  could  be  equiva- 
lent to  two  entirely  different  quantities  was  in  itself  para- 
doxical if  the  original  definition  of  equivalent  was  retained. 
These  difficulties  seem  not  to  have  been  apparent  to  Wol- 
laston. He  continued  his  determinations  of  equivalents, 
and  during  this  time  a  fusion  of  the  ideas  of  equivalent 
and  atomic  weight  took  place  unconsciously.  As  neither 
of  these  ideas  was  then  definite,  as  to  each  of  a  number  of 
elements  a  number  of  atomic  weights  could  be  assigned, 
and  almost  as  many  equivalents,  the  succeeding  period  in 
the  history  of  chemistry  was  one  of  great  confusion,  and  it 
finally  became  evident  that  some  new  idea  or  ideas  must 
be  introduced,  if  a  firm  foundation  for  the  science  was  to 
be  reached. 

Determinations  by  Berzelius. — Before  the  necessary  new 
ideas  were  introduced,  the  methods  at  hands  were  em- 
ployed to  the  full  extent.  All  known  compounds  of  any 
given  element  were  compared  with  each  other,  and  a 
number,  that  best  satisfied  the  facts,  finally  selected,  to 
represent  the  equivalent  of  the  element,  or  its  atomic 
weight  as  it  was  called  by  others.  Berzelius  attacked  the 
subject  most  successfully.  He  laid  down  rules,  by  the 
aid  of  which,  according  to  him,  the  number  of  atoms  of 
an  element  contained  in  a  compound  could  be  determined, 
and  hence  also  its  atomic  weight.  Then,  by  more  careful 
analyses  than  had  been  previously  made,  the  atomic 
weights  or  equivalents  of  all  the  elements  were  deter- 
mined. A  large  number  of  these  determinations  depended 
upon  chemical  rules,  similar  to  the  following,  given  by 
Berzelius : — 

If  an  element  forms  several  oxides,  and  the  quantities 
of  oxygen  contained  in  them,  as  compared  with  a  fixed 


COMBINING  NUMBERS,  ETC.  27 

quantity  of  the  element,  are  to  each  other  as  1 :  2,  then  it 
is  to  be  concluded  that  the  first  compound  consists  of  one 
atom  of  the  element  and  one  atom  of  oxygen;  the  second, 
of  one  atom  of  the  element  and  two  atoms  of  oxygen  (or 
two  atoms  of  the  element  and  four  atoms  of  oxygen).    If 
the  ratio  is  2:3,  then  the  first  compound  consists  of  one 
atom  of  the  element  and  two  atoms  of  oxygen;  the  second 
of  one  atom  of  the  element  and  three  atoms  of  oxygen,  etc. 
This  rule  covers  those  cases  in  which  it  is  required  to 
determine  the  atomic  weight  of  an  element  by  a  study  of 
its  oxides.    Other  rules  were  given,  in  which  sulphur  com- 
pounds, etc.,  were  made  the  basis  of  calculation. 

It  will  be  observed  that,  although  in  these  rules  the 
oxygen  and  sulphur  are  taken  as  the  elements,  the  num- 
ber of  whose  atoms  varies,  the  other  elements  might  just 
as  well  be  taken,  and  the  atomic  weights  obtained  would 
then  be  entirely  different.  An  example  will  make  this 
clear:  Mercury  combines  with  oxygen  in  two  propor- 
tions. In  the  first  compound,  8  parts  of  oxygen  are 
combined  with  199.8  parts  of  mercury;  in  the  second,  16 
parts  of  oxygen  are  combined  with  199.8  parts  of  mercury. 
Adopting  the  rule  above  laid  down,  we  should  conclude 
that  in  the  first  compound  1  atom  of  mercury  is  combined 
with  1  atom  of  oxygen,  and,  in  the  second,  1  atom  of  mer- 
cury with  2  atoms  of  oxygen.  If,  then,  8  is  the  atomic 
weight  of  oxygen,  199.8  is  the  atomic  weight  of  mercury. 
But  if,  on  the  other  hand,  the  quantity  of  oxygen  is 
regarded  as  remaining  fixed,  and  that  of  the  mercury  as 
varying,  then  we  should  have  in  the  first  compound  8 
parts  of  oxygen  combined  with  199.8  parts  of  mercury, 
and,  in  the  second,  8  parts  of  oxygen  combined  with  99.9 
parts  of  mercury ;  and,  by  a  similar  process  of  reasoning, 
the  conclusion  could  be  drawn  that  the  first  compound 
contains  2  atoms  of  mercury  to  1  atom  of  oxygen,  and 
the  second,  1  atom  of  mercury  to  1  atom  of  oxygen,  and 
thus  we  should  obtain  99.9  as  the  atomic  weight  of  mer- 
cury instead  of  199.8  as  found  above.  Berzelius  had 
made  certain  observations  on  chemical  compounds  upon 
which  he  based  his  rules;  but,  as  we  shall  see,  these  obser- 
vations were  not  sufficient. 

Another  difficulty  presented  itself  in  the  case  of  those 
elements  that  combine  only  in  one  proportion  with  oxygen. 
What  should  decide  in  regard  to  the  number  of  atoms  of 


28       PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

oxygen  contained  in  a  compound  of  such  an  element? 
Here  speculation  was  the  only  aid,  and  it  often  led  to  false 
results. 

The  Principle  of  Substitution  employed  in  the  Deter- 
mination of  Atomic  Weights. — The  researches  of  Berzelius 
added  much  to  the  knowledge  of  the  combining  weights 
of  the  elements,  and  the  determinations  made  by  him  un- 
doubtedly rested  upon  a  firmer  basis  than  the  determina- 
tions made  previously.  He  made  the  fullest  and  most 
logical  use  of  purely  chemical  means  that  could  be  made 
at  the  time.  Subsequently,  however,  a  fact  was  discovered 
in  connection  with  chemical  compounds  that  proved  of 
great  value  in  simplifying  the  consideration  of  chemical 
phenomena,  and  also  aided  materially  in  the  solution  of 
the  problem  of  the  determination  of  atomic  weights.  This 
is  substitution.  A  brief  explanation  will  suffice  here  to 
show  the  connection  between  this  subject  and  the  problem 
under  discussion.  It  has  been  found  that  certain  elements 
have  the  power  of  entering  into  compounds,  driving  out 
some  of  the  constituents.  For  instance,  water  contains  two 
atoms  of  hydrogen  and  one  of  oxygen ;  if  potassium  is 
allowed  to  act  upon  water,  a  portion  of  the  hydrogen  is 
given  off,  and  a  new  compound  containing  both  potassium 
and  hydrogen,  in  addition  to  the  oxygen,  is  formed.  If  now 
potassium  is  allowed  to  act  further  upon  this  new  compound, 
the  hydrogen  contained  in  it  is  driven  out,  and  potassium 
enters.  Thus  we  obtain  from  water,  by  substituting  potas- 
sium for  its  hydrogen,  a  compound  containing  two  atoms  of 
potassium  and  one  atom  of  oxygen.  This  kind  of  action  is 
called  substitution. 

To  show  how,  by  taking  into  account  transformations  of 
this  kind,  conclusions  of  importance  may  be  drawn  with 
reference  to  atomic  weights,  one  example  will  suffice.  It 
has  been  seen  that  the  chief  difficulty  in  determining  atomic 
weights  or  equivalents  by  chemical  means  lies  in  the  lack 
of  data  for  estimating  the  number  of  atoms  of  an  element 
contained  in  any  given  compound.  Take  the  case  of  marsh - 
gas.  In  it  1  part  by  weight  of  hydrogen  is  combined  with 
3  parts  of  carbon,  and,  as  above  stated,  from  this  fact  the 
conclusion  might  be  drawn  that  the  atomic  weight  of  car- 
bon is  3.  If,  however,  it  can  by  any  means  be  proved  that 
the  number  of  atoms  of  hydrogen  contained  in  the  gas  is 


COMBINING  NUMBERS,  ETC.  29 

greater  than  one,  the  conclusion  would  require  modifica- 
tion. By  means  of  the  process  of  substitution  this  can  be 
proved,  or  at  least  it  can  be  proved  that  the  hydrogen  con- 
tained in  marsh-gas  can  be  subdivided,  and,  hence,  if  we 
accept  the  atomic  hypothesis,  it  follows  that  the  compound 
contains  in  its  smallest  part  more  than  one  atom  of  hydro- 
gen. By  allowing  chlorine  to  act  upon  the  marsh-gas 
a  portion  of  the  hydrogen  is  removed,  and  a  compound 
containing  hydrogen  and  chlorine  is  formed.  This  new 
compound  treated  with  chlorine  again  gives  up  a  portion 
of  its  hydrogen,  and  takes  up  chlorine  in  its  place.  This 
operation  can  be  repeated  four  times,  and  thus  finally  a 
compound  is  obtained  which  contains  only  carbon  and  chlo- 
rine. Each  time  the  same  proportion  of  hydrogen  is  given 
up,  and  an  equivalent  quantity  of  chlorine  takes  its  place. 
Thus  it  is  plain  that  the  hydrogen  originally  contained  in 
marsh-gas  is  divisible  into  four  parts,  and  it  follows  that 
there  are  at  least  four  atoms  of  hydrogen  contained  in 
marsh-gas — a  conclusion  that  could  not  possibly  be  reached 
by  the  aid  of  the  methods  heretofore  considered.  If  now 
that  quantity  of  carbon  which  is  in  combination  with  four 
atoms  of  hydrogen  is  assumed  to  be  one  atom  (and,  by  a 
consideration  of  the  whole  list  of  carbon  compounds,  this 
step  is  justified),  then  the  atomic  weight  of  carbon  is  12. 
The  method,  thus  briefly  illustrated,  is  capable  of  appli- 
cation to  some  extent,  but  not  to  such  an  extent  as  to  ren- 
der it  a  general  method  for  the  determination  of  atomic 
weights. 

Consideration  of  Chemical  Decompositions  for  the  purpose 
of  determining  Atomic  Weights. — One  other  method  of  rea- 
soning must  be  referred  to  as  having  been  employed,  either 
for  the  purpose  of  furnishing  proofs  of  the  correctness  of 
atomic  weights  determined  by  other  means  or  for  the  direct 
determination  of  these  weights.  An  example  will  best 
make  this  matter  clear.  It  is  desired  to  know,  for  instance, 
how  many  atoms  of  hydrogen  are  combined  with  nitrogen 
in  ammonia ;  or,  having  by  the  preceding  method  reached 
the  conclusion  that  this  number  is  3,  we  wish  to  test  the 
conclusion  by  other  observations.  By  treating  nitric  acid 
(which  we  may  suppose  to  contain  one  atom  of  hydrogen 
to  every  atom  of  nitrogen)  with  hydrogen  ammonia  is  ob- 
tained. Now,  it  is  found  that  wfoep  a  given  quantity  of 


30      PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

nitric  acid  is  converted  into  ammonia,  in  the  resulting  am- 
monia three  times  as  much  hydrogen  is  combined  with  the 
same  quantity  of  nitrogen.  Further,  ammonia  combines 
directly  with  a  number  of  compounds,  and  an  examination 
of  the  quantity  of  hydrogen  contained  in  this  ammonia 
shows  that  it  must  necessarily  be  represented  by  three  or 
some  multiple  of  three  atoms  of  hydrogen.  Thus,  a  study 
of  the  various  cases  in  which  ammonia  is  either  formed  or 
destroyed,  or  enters  into  combination,  always  shows  that 
the  quantity  of  ammonia  thus  playing  a  part  must  contain 
three  or  some  multiple  of  three  atoms  of  hydrogen  ;  and 
hence  it  appears  that  in  ammonia  at  least  three  atoms  of 
hydrogen  are  combined  with  each  atom  of  nitrogen. 


The  methods  which  have  thus  been  briefly  described 
comprise  all  at  our  command  for  the  determination  of 
atomic  weights  dependent  upon  purely  chemical  processes. 
Consider  these  methods  as  we  may,  it  is  obvious  that  they 
are  inadequate  to  the  accomplishment  of  their  object.  The 
determinations  may,  indeed,  be  made,  but  there  must  always 
remain  a  doubt  concerning  the  result.  If,  then,  the  prob- 
lem can  be  approached  from  an  entirely  different  point  of 
view,  this  doubt  will  be  reduced  to  a  minimum,  if  it  is 
found  that  the  results  first  obtained  assert  themselves  as 
correct  in  the  second  instance.  Before  passing,  however, 
to  the  presentation  of  new  methods  for  making  these  deter- 
minations, it  will  be  well  to  apply  the  knowledge  thus  far 
gained  in  fixing  as  definitely  as  possible  the  conceptions 
of  elements  and  compounds. 

Elements. — An  element,  strictly  speaking,  is  a  substance 
that  cannot  by  any  possible  means  be  decomposed  into  kinds 
of  matter  that  are  unlike  in  their  chemical  properties.  This 
definition  presupposes  a  knowledge  of  all  possible  means  of 
decomposing  substances.  Until  we  are  positive  that  we  are 
acquainted  with  all  these  means,  we  cannot  be  positive  in 
regard  to  the  existence  of  a  single  element.  But  it  is  plain 
that  to  assert  the  possession  of  this  knowledge  would  be 
in  the  highest  degree  presumptuous.  We  can  then  never 
assert  positively  that  any  given  substance  is  an  element; 
we  can  only  say  that,  the  means  at  our  command  being 
insufficient  to  bring  about  the  decomposition  of  a  given 


COMBINING  NUMBERS,  ETC.  31 

substance,  we  regard  this  substance  as  an  element  until 
the  time  shall  arrive  when,  new  means  being  given,  it 
shall  be  shown  to  be  a  compound.  Numerous  instances 
of  the  change  of  opinion  concerning  the  elementary  char- 
acter of  different  substances  might  be  adduced,  prominent 
among  which  are  the  metals  of  the  alkalies,  the  oxides  of 
which  were  for  a  time  regarded  as  elements;  chlorine, 
which  was  regarded  as  a  compound  body  until  it  was 
shown  that  it  cannot  be  decomposed,  etc.  etc.  Thus  the 
number  of  elements,  as  stated  at  any  given  time,  is  entirely 
dependent  upon  the  state  of  chemical  analysis  at  that  time, 
and  is  never  an  expression  of  an  absolute  fact.  At  present 
the  number  of  elements  known  is  70,  including  two  or  three 
doubtful  cases.  In  other  words,  about  70  distinct  kinds  of 
matter  have  been  recognized. 

The  atoms  that  make  up  an  elementary  substance  must 
necessarily  be  of  the  same  kind.  Accepting,  then,  the 
existence  of  atoms,  an  element  may  be  defined  as  a  sub- 
stance made  up  of  atoms  of  the  same  kind  ;  and  we  shall 
see  that  the  definition  of  an  atom,  that  will  be  given  further 
on,  makes  this  definition  of  an  element  a  strict  one  in 
every  respect. 

Compounds. — Observation  shows  us  the  existence  of  at 
least  two  varieties  of  compound  substances.  To  only  one 
of  these,  however,  is  the  name  compound  strictly  appli- 
cable, and  then  the  name  signifies  a  chemical  compound. 
To  the  other  class  various  names  are  applied,  according  to 
the  nature  of  the  substance,  such,  for  instance,  as  mechan- 
ical mixture,  solution,  alloy,  etc.  Between  mechanical  mix- 
tures and  true  chemical  compounds  there  are  generally 
such  differences  that  they  can  be  distinguished  with  com- 
parative ease. 

1.  One  of  the  most  prominent  characteristics  of  chemi- 
cal compounds  is  the  possession  of  properties  which  differ 
entirely  from  those  of  their  constituents.  Hydrogen,  an 
inflammable  gas,  and  oxygen,  a  gas  and  energetic  sup- 
porter of  combustion,  combine  to  form  a  liquid,  water, 
which  is  not  inflammable  and  does  not  support  combustion. 
Hydrochloric  acid,  a  gas  that  turns  vegetable  blues  red, 
and  ammonia,  a  gas  that  turns  vegetable  reds  blue,  unite 
to  form  sal-ammoniac — a  solid  that  is  without  influence 
upon  vegetable  colors.  Chlorine,  a  gas,  and  mercury,  a 


32       PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

liquid,  give  a  solid  with  none  of  the  characteristic  proper- 
ties of  either.  The  number  of  these  examples  might  be 
increased  indefinitely,  and  in  each  case  a  similar  result 
would  be  reached. 

2.  Generally  the  constituents  of  a  chemical  compound 
cannot  be  separated  from  each  other  by  mechanical  means ; 
but  the  application  of  heat,  light,  electricity,  or  chemical 
action  is  necessary. 

3.  The    constituents  of  a  compound   are   combined   in 
fixed  proportions   by  weight.     If  these   constituents   are 
brought  together  without  reference  to  their  quantities,  and 
the  proper  condition  be  brought  about  to  cause  combina- 
tion, a  definite   quantity  of  one  combines  with  a  definite 
quantity  of  the  other;  and,  if  the  quantity  of  either  pres- 
ent is  in  excess  of  the  fixed  quantity  necessary  for  the 
formation  of  the  compound,  this  excess  will  remain  un- 
combined  after  combination  has  taken  place.     The  pro- 
portions can  be  varied  only  to  a  very  limited  extent,  and 
then  not  gradually,  but  according  to  a  fixed  rule.     This  is 
the  fact  that  above  all  others  enables  us  to  assert  positively 
that  a  given  substance  is  or  is  not  a  chemical  compound. 

Mechanical  Mixtures. — If  oxygen  and  nitrogen  are 
brought  together,  a  homogeneous  mixture  of  the  two  is 
formed,  and  this  possesses  the  properties  of  both  oxygen 
and  nitrogen ;  such  a  mixture,  for  instance,  is  the  atmos- 
phere of  the  earth.  Many  solids  may  be  mixed  in  various 
ways,  but  no  matter  how  finely  they  may  be  divided,  nor 
how  intimately  they  may  be  mixed,  provided  chemical 
combination  does  not  take  place,  the  constituents  of  the 
mixture  can  be  separated  by  mechanical  means,  and  the 
mixture  possesses  all  the  original  properties  of  its  constitu- 
ents. In  both  these  cases,  further,  the  most  varied  quan- 
tities of  the  substances  may  be  employed,  and,  under  the 
same  conditions,  the  mixtures  will  be  formed  just  as  readily 
with  one  proportion  as  with  another. 

Solutions  and  Alloys. — On  the  other  hand,  those  com- 
pounds which  are  known  under  the  names  of  solutions 
and  alloys  are  more  closely  allied  to  chemical  compounds. 
Gases,  liquids,  or  solids  may  exist  in  the  state  of  solution, 
that  is,  in  combination  with  some  liquid  body,  and  to  all 
appearance  themselves  in  the  liquid  form.  The  external 
properties  of  one  of  the  constituents  are  no  longer  recog- 


COMBINING  NUMBERS,  ETC.  33 

nizable,  and  they  are,  indeed,  in  part  lost.  A  gas  loses  its 
elasticity  when  dissolved  in  a  liquid.  A  solid  loses  the 
cohesion  which  before  held  its  particles  together.  Two 
liquids  combined  in  this  way  lose  some  of  their  original 
properties.  In  all  these  instances  the  action  between  the 
particles  of  the  dissolved  substances  and  the  particles  of 
the  solvents  is  greater  in  its  effect  than  the  cohesion  that 
originally  held  together  the  particles  of  the  solid  or  liquid, 
or  the  expansive  force  between  the  particles  of  the  gas. 
Further,  there  are  the  alloys  or  compounds  of  two  or  more 
metals.  These  alloys  present  the  appearance  of  perfectly 
homogeneous  substances,  but,  nevertheless,  possess  most  of 
the  properties  of  the  constituents.  Here,  too,  the  cohesion 
exerted  between  the  particles  of  the  original  substances  is 
modified  when  the  substances  are  brought  together. 

A  careful  examination  of  the  above-mentioned  cases 
shows  that  there  is  generally  a  limit  to  the  action.  Sub- 
stances that  are  soluble  in  water  are  not  usually  soluble  to 
an  unlimited  extent ;  on  the  contrary,  for  any  given  tem- 
perature, the  quantity  of  the  substance  that  can  be  dis- 
solved is  fixed.  But,  between  this  fixed  quantity  and  the 
smallest  possible  quantity  of  the  substance  all  proportions 
are  equally  well  dissolved.  Some  liquids  mix  with  each 
other  in  all  proportions,  a  perfectly  homogeneous  liquid 
being  the  result.  Others  dissolve  each  other  to  only  a 
limited  extent,  the  limits  being,  as  in  the  case  of  solids  and 
liquids,  fixed  for  any  given  temperature. 

The  subject  of  solution  has  received  much  attention  and 
is  still  under  investigation,  and  some  interesting  laws  have 
been  discovered  governing  the  conduct  of  dissolved  sub- 
stances. From  what  has  been  learned  it  appears  most 
probable  that,  in  many  cases  at  least,  chemical  action  takes 
place  between  a  dissolved  substance  and  a  solvent.  The 
subject  will  be  more  fully  treated  further  on. 


It  is  plain,  from  the  foregoing,  that  chemical  compounds 
and  elements  are  the  only  substances,  the  study  of  which 
can  lead  to  definite  conclusions  concerning  chemical  action. 
Let  us  now  return  to  that  fundamental  problem  of  chem- 
istry— the  determination  of  atomic  weights.  As  it  has 
been  shown  that  results,  reached  by  the  methods  already 
given,  must  necessarily  be  uncertain,  we  may  now  approach 
the  subject  from  another  side. 


34       PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 


CHAPTER  III. 

EXAMINATION    OF    GASEOUS    ELEMENTS   AND   COMPOUNDS. 

THE  methods  of  investigation  applicable  to  substances 
differ  according  to  the  state  of  aggregation  of  the  substances. 
Gases  possess  certain  properties  that  solids  and  liquids  do 
not  possess,  and  solids  and  liquids  have  certain  general 
properties  that  gases  have  not.  The  study  of  substances  in 
the  form  of  gas  or  vapor  has  led  to  most  important  results 
of  lasting  influence  upon  the  science. 

Investigations  of  Gay  Lussac. — In  the  year  1808  Gay 
Lussac  and  Humboldt  discovered  the  fact  that  when  hy- 
drogen and  oxygen  combine  to  form  water  they  combine 
in  the  proportion  of  2  volumes  of  hydrogen  to  1  volume  of 
oxygen.  The  simplicity  of  this  relation  induced  Gay  Lussac 
to  take  up  the  study  of  other  gaseous  substances,  with  the 
object  of  determining  whether  similar  relations  exist  be- 
tween the  volumes  of  other  combining  gases.  His  research 
enabled  him  soon  after  to  deduce  the  following  law  of  com- 
bination by  volumes: 

When  two  or  more  gaseous  substances  combine  to  form 
a  gaseous  compound,  the  volumes*  of  the  individual  con- 
stituents as  well  as  their  sum  bear  a  simple  relation  to 
the  volume  of  the  compound. 

Thus,  when  hydrogen  and  chlorine  unite  to  form  hydro- 
chloric acid,  1  volume  of  hydrogen  and  1  volume  of  chlorine 
form  2  volumes  of  hydrochloric  acid  gas.  Two  volumes 
of  hydrogen  and  1  volume  of  oxygen  give  2  volumes 
of  water- vapor;  2  volumes  of  nitrogen  and  1  volume  of 
oxygen  give  2  volumes  of  nitrous  oxide.  Further,  3  vol- 
umes of  hydrogen  and  1  volume  of  nitrogen  give  2  volumes 
of  ammonia,  etc. 

On  comparing  this  result  with  that  already  obtained  by 

*  In  all  cases,  where  the  volumes  of  gases  are  compared,  the  gases 
are,  of  course,  supposed  to  be  under  the  same  conditions  of  pressure 
and  temperature. 


GASEOUS  ELEMENTS  AND  COMPOUNDS.         35 

Dalton,  and  making  use  of  the  atomic  hypothesis,  accord- 
ing to  which  combination  between  elements  takes  place 
between  their  atoms,  it  will  be  seen  that  a  simple  relation 
must  exist  between  the  volumes  of  gases  and  the  relative 
number  of  atoms  contained  in  these  volumes.  This  we 
may  express  in  general  terms  as  follows : — 

The  number  of  atoms  contained  in  a  given  volume  of 
a  gaseous  substance  bears  a  simple  relation  to  the  number 
of  atoms  contained  in  the  same  volume  of  other  gaseous 
substances. 

But  this  plainly  furnishes  no  foundation  for  the  deter- 
mination of  atomic  weights,  inasmuch  as  we  have  no  means 
of  fixing  the  value  of  the  "simple  ratio,"  and  without  this 
we  cannot  determine  the  relative  number  of  atoms  con- 
tained in  a  given  volume  of  gas.  We  know  that  2  vol- 
umes of  hydrogen  combine  with  1  volume  of  oxygen,  and 
we  know  that  2  parts,  by  weight,  of  hydrogen  combine 
with  16  parts,  by  weight,  of  oxygen.  Further,  according 
to  the  atomic  hypothesis,  a  certain  number  of  atoms  of 
hydrogen  of  fixed  weight  combine  with  a  certain  number 
of  atoms  of  oxygen  of  fixed  weight,  and  these  numbers 
bear  a  simple  relation  to  each  other ;  hence,  the  relation 
between  the  number  of  atoms  of  hydrogen  in  the  2  vol- 
umes and  the  number  of  atoms  of  oxygen  in  the  1  volume 
must  be  a  simple  one,  but  the  facts  do  not  furnish  us  with 
the  data  necessary  to  enable  us  to  state  what  this  relation 
is ;  without  further  aid,  either  from  new  facts  or  specula- 
tions, we  cannot  say  what  the  atomic  weights  of  these 
elements  are. 

Avogadro's  Views. — The  numbers  expressing  the  specific 
gravities  of  gases  or  vapors  are  those  numbers  that  express 
the  relative  weights  of  equal  volumes  of  these  gases  or 
vapors.  Hence,  it  is  but  restating,  in  another  form,  the 
law  above  laid  down,  to  say  that  the  specific  gravities  of 
gaseous  bodies  bear  a  simple  relation  to  the  atomic  weights 
of  these  bodies.  The  force  of  this  statement  will  readily 
be  recognized  on  comparing  the  specific  gravities  of  some 
elementary  gases  with  the  atomic  weights  of  the  same  ele- 
ments determined  by  chemical  means.  The  atomic  weights, 
as  determined  by  chemical  means,  however,  differ  accord- 
ing to  the  method  employed  in  the  determination ;  but 
the  difference  being  that  between  one  number  and  some 


36       PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

multiple  of  that  number,  it  is  immaterial  which  of  these 
numbers  we  employ  for  the  purpose  of  the  comparison. 
Let  us,  then,  take  the  first  of  those  determined.  The 
following  table  hardly  needs  explanation.  The  numbers 
of  the  second  column  (d)  represent  the  specific  gravities 
of  the  elements  in  the  form  of  gas  or  vapor ;  the  fourth 
column  contains  the  ratios  between  the  atomic  weights 

(A)  and  d  =- 


Element. 

d. 

A. 

A 
T 

Hydrogen 
Chlorine  . 
Bromine  . 
Iodine 
Oxygen    . 
Sulphur  . 

0.0692 
2.440 
5.54 
8.716 
1.10563 
223 
568 
9.08 
0.9713 
4.50 
10.6 
7.03 
394 

1 
354 
799 
126.5 
8 
16 
39.4 
625 
14 
31 
74.9 
999 
55.9 

1445 
14.51 
14.42 
14.51 
7.24 
7,17 
694 
6.88 
14.41 
689 
7.07 
14.21 
1419 

Tellurium 
Nitrogen  . 
Phosphorus 
Arsenic    . 
Mercury  . 
Cadmium 

From  this  it  appears  that  the  relation  between  the  spe- 
cific gravity  and  the  atomic  weight  of  seven  of  these  thir- 
teen elements  is  the  same,  being  expressed  by  a  number 
varying  but  little  from  14.4.  In  the  case  of  the  six  remain- 
ing elements  of  the  list  the  relation  is  also  virtually  the 
same,  about  7.1.  And,  in  the  latter  case,  the  ratio  is 
expressed  by  a  number  half  as  great  as  the  first. 

A  consideration  of  these  relations  led  Avogadro,*  in 
1811,  to  propose  an  hypothesis,  which,  if  it  is  well  founded, 
must  prove  of  the  greatest  service  in  simplifying  the  prob- 
lem of  determining  the  atomic  weights — at  least  of  those 
elements  of  which  gaseous  compounds  are  known.  It  will 
be  seen  that,  if  in  the  above  table  the  atomic  weights 
of  oxygen,  sulphur,  selenium,  tellurium,  phosphorus,  and 

A 

arsenic  are  doubled,  the  ratio  -^  for  all  the  elements  in  the 


*  In  1814  Ampere  made  a  similar  suggestion. 


GASEOUS  ELEMENTS  AND  COMPOUNDS.         37 

list  will  be  the  same  constant  number,  viz.,  about  14.4. 
But  the  atomic  weights  above  given  have  been  determined 
purely  empirically,  and  we  are  as  much  justified  in  con- 
sidering the  larger  numbers  the  true  atomic  weights  as 
we  are  in  accepting  the  ones  given.  If  this  change  is 
made,  then,  for  the  above  thirteen  elements,  the  following 
statement  will  be  true:  The  atomic  weights  are  to  each 
other  as  the  specific  gravities  of  the  vapors.  An  exam- 
ination of  compound  gaseous  substances  showed  further 
that  a  simple  relation  also  exists  between  their  specific 
gravities  and  the  numbers  expressing  the  sum  of  the  atomic 
weights  of  the  constituents,  these  sums  being  to  each  other 
as  the  specific  gravities.  Avogadro's  hypothesis  to  account 
for  these  relations  may  be  stated  in  the  following  words : — 
All  gases  and  vapors,  without  exception,  contain,  in 
the  same  volume,  the  same  number  of  ultimate  particles 
or  molecules. 

The  molecules  were  not  considered  to  be  identical  with 
the  atoms,  and  it  is  well  here  to  make  the  distinction  be- 
tween the  two  as  clear  as  possible.  Molecules  of  com- 
pounds, as  understood  by  Avogadro,  and  as  understood  at 
present^  are  the  hypothetical  smallest  particles  of  these 
compounds.  The  molecule  of  water  is  the  smallest  particle 
of  water  that  can  exist  as  water.  As  water,  however,  is 
composed  of  two  elements,  of  course,  the  smallest  particle 
of  water  must  necessarily  still  be  divisible  into  these  con- 
stituents. The  component  parts  of  molecules  are  called 
atoms,  and  these  are  indivisible.  In  the  case  of  water,  the 
molecule  has  the  same  composition  as  the  mass  of  the  com- 
pound ;  but,  as  will  be  shown,  this  molecule  of  water  con- 
sists of  two  atoms  of  hydrogen  and  one  atom  of  oxygen. 
The  holding  together  of  the  two  atoms  is  a  chemical  act. 
That  which  holds  the  molecule  together  is  called  cohesion. 

Now,  there  are  good  reasons,  which  will  be  considered 
below,  for  believing  that,  in  their  internal  structure,  ele- 
mentary substances  are,  in  some  respects,  analogous  to  com- 
pounds, and  this  belief  was  made  a  fundamental  condition 
of  Avogadro's  hypothesis.  According  to  this,  it  is,  in  most 
cases,  impossible,  by  purely  mechanical  means,  to  subdivide 
an  element  so  far  as  to  reach  its  atoms ;  but,  if  we  suppose 
it  divided  as  far  as  possible  by  such  means,  we  reach,  as  in 
the  case  of  compounds,  the  molecule  of  the  element,  which 
is  the  smallest  particle  of  the  element  that  can  exist  and 

3 


38       PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

play  the  part  of  the  element.  This  molecule,  however, 
usually  consists  of  atoms  that  are  held  together  by  a  chem- 
ical act,  and  can  hence  be  separated  only  by  means  other 
than  mechanical. 

From  these  considerations  definitions  of  the  terms  atom 
and  molecule  follow : — 

A  molecule  is  the  smallest  particle  of  a  compound  or 
element  that  is  capable  of  existing  in  a  free  state.  A 
breaking  up  of  the  molecule  necessitates  the  destruction 
of  the  characteristic  properties  of  the  compound,  and 
almost  always  of  those  of  the  element. 

Atoms  are  the  indivisible  constituents  of  molecules. 
They  are  the  smallest  particles  of  elements  that  take  part 
in  chemical  reactions,  and  are,  for  the  greater  part,  in- 
capable of  existence  in  the  free  state,  being  generally 
found  in  combination  with  other  atoms,  either  of  the 
same  kind  or  of  different  kinds. 

And  now  the  significance  of  the  definitions  of  elements  and 
compounds  given  above  will  be  recognized,  viz.,  an  element 
is  a  substance  made  up  of  atoms  of  the  same  kind ;  a  com- 
pound is  a  substance  made  up  of  atoms  of  different  kinds. 
Recognizing  thus  fully  the  distinction  between  atoms 
and  molecules,  we  are  prepared  further  to  follow  the  rea- 
soning of  Avogadro. 

The  experiments  of  Gay  Lussac  had  already  proved  that, 
under  the  influence  of  heat,  all  gases  expand  in  the  same 
proportion  for  the  same  increase  in  temperature,  and  di- 
minish in  volume  to  the  same  extent  for  the  same  decrease 
of  temperature.  Further,  Mariotte  and  Boyle  had  shown 
that  all  gases  conduct  themselves  in  the  same  way  under 
the  influence  of  increased  or  decreased  pressure ;  that  for 
the  same  increase  or  decrease  of  pressure  the  resulting  de- 
crease or  increase  of  volume  is  the  same  for  the  same  vol- 
ume of  all  gases.  These  facts  considered  independently 
would  lead  to  the  suspicion  that  all  gases  possess  a  similar 
internal  structure,  and  the  simplest  hypothesis  to  account 
for  this  is  the  hypothesis  of  Avogadro — that  the  same  vol- 
umes of  all  gaseous  bodies  contain  the  same  number  of 
molecules.  This  subject  has  been  treated  exhaustively 
from  a  purely  physical  point  of  view.  The  mechanical 
theory  of  gases  being  accepted,  it  has  been  shown  that  the 
hypothesis  of  Avogadro  follows  as  a  necessary  consequence ; 
and  then,  by  a  purely  mathematical  process  of  reasoning, 


GASEOUS  ELEMENTS  AND  COMPOUNDS.        39 

it  has  been  shown  that  the  hypothesis  is  an  absolute  neces- 
sity. A  discussion  of  the  subject  in  the  direction  indicated 
cannot  be  taken  up  here. 

AP  a  grand  result  of  the  investigations  that  have  been 
made  on  the  internal  structure  of  gases,  it  may  be  stated 
that  Avogadro's  hypothesis  has  throughout  asserted  its 
correctness,  and  it  has  long  been  recognized  as  of  funda- 
mental importance  in  the  science  of  chemistry.  It  is  at 
present  almost  universally  accepted  by  chemists,  and  is 
generally  referred  to  as  a  law. 

Determination  of  Molecular  Weights. — What,  then,  do 
we  gain  by  accepting  the  hypothesis  ?  It  is  plain  that  if 
equal  volumes  of  all  gases  contain  the  same  number  of 
molecules  we  have  a  means  given  us  at  once  for  ascertain- 
ing the  relative  weights  of  these  molecules.  We  have 
merely  to  determine  the  relative  weights  of  equal  volumes 
of  the  gases,  and  the  numbers  obtained  will  bear  the  same 
relations  to  one  another  as  the  molecular  weights.  Then 
accepting  the  weight  of  some  molecule  as  a  standard,  and 
expressing  the  weights  of  the  others  in  terms  of  this 
standard,  the  molecular  weights  are  determined.  Let  us, 
for  example,  take  hydrochloric  acid  as  the  standard  mole- 
cule. As  this  compound  contains  35.4  parts  by  weight  of 
chlorine  to  1  of  hydrogen,  the  smallest  figure  by  which 
we  can  represent  its  molecular  weight,  without  repre- 
senting the  weight  of  hydrogen  by  a  figure  less  than  1,  is 
36.4.  By  further  study  of  hydrochloric  acid  it  is  found 
that  no  facts  are  known  that  require  us  to  select  a  figure 
larger  than  36.4  for  its  molecular  weight.  We  accordingly 
accept  this  as  the  molecular  weight  of  hydrochloric  acid. 
We  now  determine  the  specific  gravity  of  the  gas,  and 
express  the  result  in  terms  of  air.  The  figure  is  1.247,  or, 
to  be  explicit,  if  the  weight  of  a  given  volume  of  air  is 
represented  by  1,  then  the  weight  of  the  same  volume  of 
hydrochloric  acid  gas  is  represented  by  1.247.  As,  accord- 
ing to  Avogadro's  hypothesis,  the  molecular  weights  of 
gaseous  bodies  bear  the  same  relation  to  each  other  as  their 
specific  gravities,  it  is  only  necessary  to  determine  in  one 
particular  case  the  relation  between  the  molecular  weight 
and  the  specific  gravity.  The  molecular  weight  accepted 
for  hydrochloric  acid,  for  the  reasons  above  given,  is  36.4  ; 
the  specific  gravity,  determined  by  actually  weighing  the 


40       PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

gas,  is  1.274.     The  relation  between  the  two  figures  is 

36.4 
expressed  thus,   ^  274 ==  28.57 ;    or,  calling  the  specific 

gravity  d,  and  the  molecular  weight  M,  we  have  for  hydro- 
chloric acid  -T  =  28.57,  M=dx  28.57.  Having  deter- 
mined the  relation  between  the  specific  gravity  and  the 
molecular  weight  of  one  gaseous  substance,  we  have,  how- 
ever, determined  it  for  all,  and  we  thus  have  in  our  pos- 
session a  method  for  determining  molecular  weights  that 
depends  upon  the  determination  of  the  specific  gravities  of 
gases  or  vapors.  If  the  rule  is  perfect,  and  the  figures 
obtained  by  experiment  were  absolutely  accurate,  then  by 
dividing  the  molecular  weight  of  any  gaseous  substance  by 
its  specific  gravity,  we  should  in  every  case  obtain  the 
same  quotient.  Owing  partly  to  the  imperfection  of  the 
methods  for  determining  specific  gravities  and  for  analyzing 
chemical  compounds,  the  figures  actually  obtained  do  not 
give  exactly  the  quotient  obtained  in  the  case  of  hydro- 
chloric acid.  The  average  of  the  results  is  more  nearly 
28.88,  and  as  this  is  the  figure  obtained  by  dividing  the 
molecular  weight  of  hydrogen  by  the  specific  gravity,  it  is 
the  one  commonly  given  in  stating  the  rule.  Instead  of 
being  M=d  X  28.57,  it  is  M=  d  x  28.88. 

Applying  the  rule  to  the  determination  of  molecular 
weights,  results  approximating  the  truth  are  obtained. 
These  enable  us  to  decide  whether  the  molecular  weight  is 
a  certain  figure  or  a  multiple  of  this  figure.  Chemical 
analysis  then  comes  to  our  aid  and  tells  us  exactly  what 
the  number  is.  To  illustrate  this,  take  the  case  of  water. 
We  find  by  determining  the  specific  gravity  of  water  vapor 
and  multiplying  by  28.88  that  the  molecular  weight  is 
17.99.  This  enables  us  at  once  to  decide  between  the 
various  possibilities,  9,  18,  27,  etc.  If  we  now  determine 
with  great  accuracy  the  proportions  by  weight  in  which 
hydrogen  and  oxygen  are'  combined  with  each  other  in 
water,  we  shall  be  able  to  state  exactly  what  the  molecular 
weight  of  water  is.  According  to  the  most  reliable  inves- 
tigations the  figure  is  17.96.  The  coincidence  of  the 
numbers  determined  by  the  two  methods  in  the  case  of  a 
few  elements  and  compounds  will  be  seen  on  examining  the 
subjoined  table.  The  numbers  under  M  are  those  found 


GASEOUS  ELEMENTS  AND  COMPOUNDS. 


41 


by  the  analytical  method,  that  one  of  a  series  of  multiples 
being  selected  which  agrees  most  nearly  with  the  number 
found  according  to  the  rule  M  =  28.88X  d. 


Name. 

Sp.  gravity. 
=  d. 

28.88  X  d. 

M. 

Hydrogen 

0.06926 

2 

2 

Nitrogen  . 

0.9713 

2805 

28 

Oxygen    . 

1.10563 

31.93 

31.9 

Sulphur   . 

2.23 

64.4 

63.9 

Chlorine  . 

2.45 

70.75 

70.8 

Cadmium 

3.94 

113.78 

111.7 

Phosphorus 

4.35 

125.62 

123.8 

Bromine  . 

5.54 

159.99 

159.5 

Selenium. 

568 

164.03 

157.74 

Mercury  . 

6.98 

201.58 

199.8 

Water      . 

0.623 

17.99 

17.96 

Hydrochloric  acid 

1.247 

36.11 

36.4 

Sulphur  dioxide 

2.247 

64.89 

63.9 

Ammonia 

0.597 

17.24 

17 

Phosphorus  tiichloi 

ide 

4.88 

140.93 

137.1 

Arsenic  trichloride 

6.30 

181.94 

181 

Boron  chloride 

3.942 

113.84 

117 

Marsh  -gas 

0.557 

16.08 

16 

Methyl  chloride 

1.736 

50.13 

50.7 

Chloroform 

4.20 

121.29 

119  1 

Tin  chloride     . 

9.20 

265.69 

258.9 

Silicon  chloride 

594 

171.55 

169.5 

Zinc-methyl     . 

3.29 

9502 

94.8 

Aluminium  chloride 

9.35 

270.03 

2663 

Ferric  chloride 

11.39 

328.94 

324 

Number  of  Atoms  in  the  Molecules  of  Elements. — Although 
we  are  thus  enabled  to  determine  by  a  simple  process  the 
molecular  weights  of  those  elements  which  are  gases  under 
ordinary  conditions,  or  can  be  converted  into  gases,  an 
important  part  of  the  problem — the  determination  of  the 
atomic  weights — yet  remains  to  be  solved.  If  we  knew  in 
each  case  how  many  atoms  are  contained  in  a  molecule, 
our  difficulties  would  be  at  an  end ;  but  this  we  plainly 
do  not  know  without  the  introduction  of  considerations  of 
a  different  kind  from  those  with  which  we  have  thus  far 
had  to  deal.  Taking  hydrochloric  acid  as  the  standard  in 
determining  the  molecular  weights,  and  representing  its 
molecular  weight  by  36.4,  because  that  is  the  smallest 
figure  permissible  if  the  hydrogen  in  it  is  not  to  be  ex- 


42      PRINCIPLES  OF*  THEORETICAL  CHEMISTRY. 

pressed  by  a  figure  less  than  1,  we  find  that  the  molecular 
weight  of  hydrogen,  determined  by  the  rule  of  Avogadro, 
is  2.  Now  the  part  of  hydrogen  contained  in  hydrochloric 
acid,  which  is  represented  by  1,  must  be  at  least  an  atom 
of  hydrogen.  Or,  further,  the  36.4  parts  by  weight  of 
hydrochloric  acid,  representing  the  molecule,  must  be  made 
up  of  at  least  one  atom  of  hydrogen,  weighing  1,  and  one 
atom  of  chlorine,  weighing  35.4.  But,  as  we  find  that  the 
molecular  weight  of  hydrogen  is  2,  it  follows  that  the  mole- 
cule must  be  at  least  twice  as  heavy  as  the  atom,  or  the  mole- 
cule must  contain  at  least  two  atoms.  We  may  also  reason 
as  follows,  with  reference  to  some  of  the  simpler  chemical 
compounds:  Given  hydrochloric  acid,  it  is  required  to 
know  how  many  atoms  are  contained  in  a  molecule  of 
hydrogen  and  in  a  molecule  of  chlorine.  If  in  a  certain 
volume  of  hydrogen  there  are  contained  say  100  molecules, 
then  in  the  same  volume  of  chlorine  there  are  contained 
the  same  number  of  molecules.  Now  it  is  known  that  1 
volume  of  hydrogen  combines  with  1  volume  of  chlorine. 
Two  volumes  of  hydrochloric  acid  gas  are  formed,  and, 
according  to  the  hypothesis,  these  two  volumes  in  the  case 
under  consideration  contain  200  molecules.  But  each 
molecule  of  hydrochloric  acid  must  contain  at  least  one 
atom  of  chlorine  and  one  atom  of  hydrogen ;  hence,  in 
100  molecules  of  hydrogen  and  100  molecules  of  chlorine 
there  must  be  at  least  200  atoms  of  hydrogen  and  200  atoms 
of  chlorine,  or  a  molecule  of  either  hydrogen  or  chlorine 
must  contain  at  least  two  atoms.  Further,  as  no  simpler 
compound  of  hydrogen  or  of  chlorine  is  known  than  hydro- 
chloric acid,  any  conclusions  which  we  may  draw  through 
a  consideration  of  this  compound  must  be  valid  for  all 
compounds  of  these  elements.  The  supposition  that  two 
atoms  form  the  molecule  of  hydrogen  and  that  of  chlorine  is 
in  accordance  with  all  the  facts  known  to  us,  and  we  hence 
rest  with  this  supposition.  It  must,  however,  be  distinctly 
borne  in  mind  that  no  proof  is  here  given  of  the  absolute 
number  of  atoms  contained  in  the  molecules  of  hydrogen 
and  chlorine.  We  can  only  say  that  at  least  2  atoms  must 
be  present  in  each  of  the  molecules.  There  may  be  a  much 
greater  number,,  but  the  data  permit  no  conclusion  beyond 
this  number  2. 

For  all  similar  cases  a  similar  process  of  reasoning  may 
be  employed,  and  with  the  same  results.    Whenever  1  vol- 


GASEO  us  ELEMEzrr&aaaf&mpo  UNDS.      43 


ume  of  an  elementary  gas  or  vapor  combines  with  1  vol- 
ume of  another  elementary  gas  or  vapor  to  form  2  volumes 
of  a  compound  gas  or  vapor,  we  are  justified  in  concluding 
that  each  molecule  of  these  elements  contains  two  atoms. 
The  elements  that  come  under  this  head  are  hydrogen  and 
chlorine. 

If  we  pass  to  oxygen,  we  find  a  material  difference  in 
the  conduct:  2  volumes  of  hydrogen  combine  with  1  vol- 
ume of  oxygen  to  form  2  volumes  of  water- vapor.  Let 
us  reason  as  above.  If  in  1  volume  of  oxygen  there  are 
contained  100  molecules,  then  in  2  volumes  of  hydrogen 
there  are  200  molecules.  These  300  molecules  combine  to 
form  200  molecules  of  the  compound.  Now,  in  the  mole- 
cule of  water  there  must  be  contained  at  least  1  atom  of 
oxygen  and  1  atom  of  hydrogen ;  hence  there  must  be  at 
least  200  atoms  of  oxygen  and  200  atoms  of  hydrogen. 
But  we  know  that  in  the  original  200  molecules  of  hydro- 
gen there  were  contained  400  atoms ;  hence,  in  each  mole- 
cule of  water  there  must  be  2  atoms  of  hydrogen.  Water 
is  the  simplest  compound  of  oxygen  known  to  us  (i.  e.,  it 
contains  the  smallest  relative  quantity  of  oxygen  in  the 
molecule),  and  on  this  account  we  suppose  the  molecule  of 
water  to  contain  1  atom  of  oxygen.  If,  then,  each  mole- 
cule of  water  contains  2  atoms  of  hydrogen  and  1  atom  of 
oxygen,  in  the  200  molecules  of  water  there  are  200  atoms 
of  oxygen  and  400  atoms  of  hydrogen,  and  these  are  ob- 
tained from  100  molecules  of  oxygen  and  200  molecules  of 
hydrogen.  Therefore,  each  molecule  of  oxygen,  as  well  as 
each  molecule  of  hydrogen,  contains  2  atoms. 

Another  method  of  reasoning,  starting  from  entirely 
different  facts,  also  led  Favre  and  Silbermann  to  suggest 
that  the  molecule  of  oxygen  consists  of  2  atoms.  They 
proved  that  carbon,  when  burned  in  nitrous  oxide,  evolves 
more  heat  than  when  burned  in  oxygen.  The  simplest 
interpretation  of  this  fact  is  that,  in  each  experiment,  a 
chemical  combination  is  destroyed  while  another  is  formed  ; 
and  that  the  amount  of  heat  actually  evolved  is  the  differ- 
ence between  the  amount  of  heat  disengaged  by  the  union 
of  carbon  with  oxygen  and  the  amount  of  heat  absorbed 
by  the  decomposition  of  the  oxide  of  oxygen  in  the  first 
instance,  and  of  oxide  of  nitrogen  in  the  second.  And,  if 
the  thermic  effect  is  less  with  oxygen  than  with  nitrous 
oxide,  that  is  due  to  the  circumstance  that  more  heat  is 


44      PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

absorbed  in  the  decomposition  of  the  oxide  of  oxygen  (the 
molecule  of  oxygen  O2)  than  in  the  decomposition  of  the 
molecule  of  nitrous  oxide. 

One  volume  of  nitrogen  combines  with  3  volumes  of 
hydrogen  to  form  2  volumes  of  ammonia.  Hence,  in  the 
molecule  of  ammonia  there  are  3  atoms  of  hydrogen,  and 
ammonia  being  the  simplest  compound  of  nitrogen,  we  sup- 
pose that  these  3  atoms  of  hydrogen  are  combined  with  1 
atom  of  nitrogen.  As  each  molecule  of  ammonia  contains 

1  atom  of  nitrogen,  and  as,  further,  twice  as  many  mole- 
cules of  ammonia  are  formed  as  there  were  molecules  of 
nitrogen  originally,  it  follows  that  the  molecule  of  nitrogen 
contains  at  least  2  atoms. 

By  this  means  we  are  enabled  to  determine  the  atomic 
weight  of  the  elements  mentioned,  for  if  in  their  molecules 

2  atoms  are  contained,  we  have  only  to  divide  the  molec- 
ular weight — found  by  Avogadro's  rule,  and  corrected  by 
analytical  methods — by  t\vo.     But   accepting  the  atomic 
weights   of  hydrogen,   chlorine,   bromine,   and   iodine  as 
known,  we  are  enabled  by  another  process  to  determine  the 
atomic  weights  of  such  elements  as  combine  with  these  to 
form  gaseous  compounds. 

Take,  again,  water.  We  find  by  a  comparison  of  the 
compounds  of  oxygen  that  the  molecule  of  water,  as  stated 
above,  contains  as  small  a  quantity  of  this  element  as  any 
other  compound ;  and  hence  we  suppose  this  quantity  to 
represent  1  atom.  We  first  find  the  molecular  weight  from 
the  specific  gravity  of  the  vapor.  This  is  18.  We  analyze 
the  compound,  and  find  that  it  contains  88.89  per  cent, 
oxygen,  and  11.11  per  cent,  hydrogen,  or  8  parts  of  oxygen 
to  1  part  of  hydrogen.  Therefore  in  18  parts  by  weight, 
which  represent  the  molecule,  there  are  contained  16  parts 
of  oxygen  and  2  parts  of  hydrogen.  The  atomic  weight 
of  oxygen  is  hence  16,  and  in  water  1  atom  of  oxygen  is 
combined  with  2  atoms  of  hydrogen.  In  the  same  way, 
on  comparing  the  molecular  weights  of  the  compounds  of 
nitrogen  we  find  that  the  relative  quantity  of  this  element 
contained  in  the  molecule  of  ammonia  is  as  small  as  in  any 
other.  The  molecular  weight  of  ammonia  we  find  to  be 
17.  The  analysis  shows  that  the  elements  are  combined  in 
the  proportion  of  14  parts  by  weight  of  nitrogen  to  3  parts 
by  weight  of  hydrogen.  Hence  14  is  the  atomic  weight  of 


GASEOUS  ELEMENTS  AND  COMPOUNDS.        45 

nitrogen,  and  the  molecule  of  ammonia  contains  1  atom  of 
nitrogen  and  3  atoms  of  hydrogen. 

Molecules  of  Elements  which  contain  more  or  less  than  two 
Atoms.  —  The  molecules  of  the  elements  considered  contain 
each  at  least  2  atoms.  This,  however,  is  not  true  of  the 
molecules  of  all  elements. 

Among  those  compounds  of  phosphorus  which  may  be 
looked  upon  as  containing  1  atom  of  this  element  in  the 
molecule  is  phosphine.  The  molecular  weight  of  phosphine 
is  34.  The  elements  are  contained  in  it  in  the  proportion 
of  31  parts  of  phosphorus  to  3  parts  of  hydrogen.  Hence 
31  is  the  atomic  weight  of  phosphorus.  On  the  other  hand, 
we  find  the  molecular  weight  of  phosphorus  itself  to  be  124, 
which  shows  that  at  least  4  atoms  are  contained  in  the 
molecule.  The  same  is  true  of  arsenic. 

For  reasons  similar  to  those  given  above  the  molecule  of 
mercuric  chloride  is  supposed  to  contain  1  atom  of  mer- 
cury. The  molecular  weight  of  this  compound  is  found  to 
be  270.5,  and  the  elements  are  contained  in  it  in  the  pro- 
portion of  199.8  parts  of  mercury  to  70.7  parts  of  chlorine, 
which  gives  199.8  as  the  atomic  weight  of  mercury,  and 
the  atom  of  this  element  is  combined  with  2  atoms  of 
chlorine.  The  molecular  weight  of  mercury  is  200  ;  hence, 
in  the  molecule  of  mercury  there  is  contained  but  1  atom. 
The  same  coincidence  of  atomic  and  molecular  weight  is 
noticed  in  the  case  of  cadmium  and  zinc. 

Kundt  and  Warburg  have  described  an  interesting  ex- 
periment, the  results  of  which  also  show  that  the  molecule 
of  mercury  in  all  probability  consists  of  a  single  atom. 
The  quantity  of  heat  contained  in  a  gas  is  defined  as  the 
total  energy  of  its  molecules,  and  this  energy  consists  solely 
in  translatory  motion,  if  the  molecule  is  looked  upon  as 
a  mere  material  point.  According  to  this,  it  is  a  simple 
matter  to  calculate  the  relation  between  the  specific  heat 
of  a  gas  at  constant  volume  and  the  specific  heat  at  con- 
stant pressure.  It  has  been  found  that  in  the  case  of  the 
gases  examined  the  theoretical  value  of  this  ratio  is  larger 
than  the  value  actually  observed.  If  c  represents  the 
specific  heat  at  constant  volume,  and  c'  the  specific  heat  at 

constant  pressure,  then  G~=k  represents  the   ratio  above 
c 

referred  to.  According  to  the  theory,  k  =  1.67,  whereas 
observation  gives  h  —  1.405.  In  other  words,  it  requires 

3*  ^    '    '  '"    • 

v»X     •  Mk 


Tr'NTTVPTRRTTY    I 


46      PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

more  heat  to  raise  the  temperature  of  a  gas,  the  volume 
remaining  unchanged,  than  the  theory  demands.  The  heat 
that  thus  disappears  may  be  transformed  into  an  inter- 
molecular  motion;  i.e.,  the  atoms,  composing  the  molecule, 
may  have  a  motion  relative  to  some  centre  of  gravity. 
This  motion  would  not  show  itself  as  temperature.  If  the 
molecule  of  the  gas  consists  of  1  atom,  then  the  theoretical 
and  observed  value  of  k  should  be  identical.  The  exami- 
nation of  mercury  gave  for  k  the  value  1.67,  which  is  that 
above  given  as  the  result  of  calculation.  It  is  thus  shown 
by  a  method  entirely  independent  of  chemistry,  that  the 
molecule  of  mercury  conducts  itself  like  a  material  point, 
and  this  could  only  be  the  case  if  it  consisted  of  1  atom. 

Varying  number  of  Atoms  in  the  Molecule  of  one  and  the 
same  Element. — The  specific  gravity  of  the  vapor  of  sulphur 
was  stated  in  the  above  table  (p.  41)  to  be  2.23,  and  this 
leads  to  the  molecular  weight  63.9.  Now,  it  has  been 
found  that  the  specific  gravity  of  sulphur  vapor  varies 
according  to  the  temperature  at  which  it  is  determined. 
The  determinations,  which  gave  the  number  2.23,  were 
made  at  temperatures  above  800°  C.  (860°  and  1040°). 
Other  determinations,  however,  made  below  800°  gave 
different  results.  At  524°  (Dumas)  and  508°  (Mitscher- 
lich)  the  specific  gravity  was  found  to  be  6.62  and  6.90 
respectively,  or  three  times  as  great  as  at  the  higher  tem- 
peratures. These  latter  determinations  gave  the  molecular 
weight  approximately  192,  and,  if  32  is  the  atomic  weight 
of  sulphur,  then  in  the  molecule  of  the  vapor  below  800° 
there  are  contained  6  atoms,  whereas  above  800°  there  are 
contained  only  2  atoms  in  the  molecule.  According  to 
recent  researches  by  Biltz,  the  specific  gravity  of  the  vapor 
of  sulphur  changes  gradually  up  to  800°,  and  there  is  no 
evidence  of  the  existence  of  the  molecules  S6,  S4.  At  800° 
the  value  found  points  to  the  formula  S2,  and  there  is  then 
no  change  through  an  interval  of  over  900°,  as  has  been 
shown  by  V.  Meyer  and  Biltz.  Kiecke  has  pointed  out 
that  the  results  of  Biltz  can  be  best  explained  by  supposing 
that  the  first  result  of  the  vaporization  of  sulphur  is  the 
formation  of  molecules  S8,  and  that  these  first  dissociate 
into  molecules  S6  and  S2,  the  molecules  Sg  gradually  pass- 
ing over  into  molecules  S2.  Selenium,  so  similar  to  sulphur 
in  all  other  respects,  presents  similar  phenomena,  though 
not  in  so  marked  a  degree.  Here,  too,  it  is  noticed  that 


GASEOUS  ELEMENTS  AND  COMPOUNDS.        47 

the  specific  gravity  of  the  vapor  decreases  with  an  increase 
of  temperature. 

Analogous  results  have  been  obtained  in  the  case  of 
iodine.  The  normal  specific  gravity  of  iodine  vapor  is 
8.8,  corresponding  to  the  molecular  weight  254.  It  was 
first  shown  by  Victor  Meyer  that  at  1027°  C.  the  specific 
gravity  is  reduced  to  5.8.  J.  M.  Crafts  and  F.  Meier 
showed  that  at  1468°  it  is  still  further  reduced,  becoming 
5.1.  Victor  Meyer  then  succeeded  in  reducing  it  to  4.5 
by  heating  it  to  a  higher  temperature;  and,  finally,  Crafts 
and  Meier  proved  that  by  a  further  elevation  of  tempera- 
ture the  specific  gravity  is  not  reduced  below  the  value 
4.5,  which  is  very  nearly  half  the  normal.  The  simplest 
interpretation  of  these  facts  is  this :  Under  ordinary  con- 
ditions the  molecule  of  the  vapor  of  iodine  consists  of  2 
atoms,  When  the  temperature  is  raised  there  is  a  gradual 
decomposition  of  the  molecules  into  2  atoms  each.  This 
decomposition  continues  as  the  temperature  becomes 
higher,  until  finally  all  the  molecules  are  broken  up  into 
atoms.  When  this  limit  is  reached,  no  further  decompo- 
sition being  possible,  the  specific  gravity  remains  un- 
changed, even  though  the  temperature  is  raised  still 
higher.  The  fact  that  the  reduction  in  the  specific  gravity 
stops  when  it  reaches  half  the  normal  is  especially  signifi- 
cant, as  it  furnishes  strong  evidence  of  the  presence  of  2, 
and  only  2,  atoms  in  the  molecule  of  iodine. 


The  application  of  the  above  method  to  the  determina- 
tion of  the  molecular  weights  of  elements  is  limited,  as 
only  a  few  of  these  elements  can  be  converted  into  vapor. 
Of  many  elements,  however,  compounds  are  known  that 
are  capable  of  conversion  into  vapor,  or  are  themselves 
gaseous,  and,  as  the  molecular  weights  of  these  compounds 
can  be  determined,  the  atomic  weights  of  the  elements  of 
which  they  are  made  up  can  also  be  determined.  The 
following  table  contains  a  number  of  such  compounds, 
together  with  the  specific  gravities  (d ) ;  the  products  of 
the  specific  gravities  by  the  constant  28.88  (d  X  28.88) ; 
the  molecular  weights  as  found  by  analytical  methods  (M) ; 
and,  finally,  the  relative  quantities  of  the  constituents  of 
the  compounds  contained  in  the  molecules  as  determined 
by  analysis : — 


48       PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 


Name. 

d. 

dX28.88 

M. 

Constituents. 

Aluminic  chloride 

9.35 

270 

266.3 

54.08  parts  aluminium. 

212.22 

''    chlorine. 

Aluminic  bromide 

18.6 

537.2 

532.64 

54.08 

'*     aluminium. 

478.56 

"     bromine 

Aluminic  iodide     . 

27 

780 

81332 

54.08 

'     aluminium. 

759.24 

"    iodine. 

Antimony   trichlo- 

ride . 

7.8 

225.3 

22571 

119.6 

"     antimony. 

106.11 

'*    chlorine. 

Triethylstibine  .     . 

7.44 

2148 

206.42 

119.6 

"    antimony. 

71.82 

"     carbon. 

15 

"     hydrogen. 

Antimony  trioxide 

19.79 

571.5 

57416 

478.4 

"    antimony. 

95.76 

"    oxygen. 

Arsenic  triiodide    . 

16.1 

464.9 

454.52 

74.9 

"     arsenic. 

379.62 

"     iodine. 

Arsine      .... 

2.7 

77.97 

779 

74.9 

"     arsenic. 

3 

"     hydrogen. 

Arsenic  trichloride 

6.3 

181.9 

181.01 

749 

"     arsenic. 

106.11 

'*    chlorine. 

Arsenic  trioxide     . 

13.79 

398.3  j  396.36 

299.6 

"     arsenic. 

95.76 

11    oxygen. 

Cacodyl  chloride    . 

4.56 

131.7 

140.21 

74.9 

'    arsenic. 

23.94 

'    carbon. 

6 

'     hydrogen. 

35.37 

"    chlorine. 

Cacodyl  cyanide     . 

4.65 

134.3 

130.82 

74.9 

'     arsenic. 

35.91 

'    carbon. 

14.01 

"     nitrogen. 

Bismuth  trichloride 

11.35 

327.8 

313.41 

6 

207.3 

"     hydrogen. 
"    bismuth. 

106.11 

"    chlorine. 

Boric  chloride  .     . 

3.942 

113.8 

117.01 

10.9 

"     boron. 

106.11 

"    chlorine. 

Boric  fluoride    .     . 

2.312 

66.8 

68.08 

10.9 

"     boron. 

57.18 

"    fluorine. 

Boric  bromide  .     . 

8.78 

253.5 

250.18 

10.9 

"     boron 

239.28 

"     bromine. 

Trimethylborine    . 

1.93 

55.7 

55.81 

10.9 

;    boron. 

35.91 

"    carbon. 

9 

"    hydrogen 

Cadmium  bromide  . 

9.25 

267.1 

271.22 

111.7 

"    cadmium. 

159.52 

"     bromine. 

Marsh  -gas     .     .     . 

0.557 

16.1 

15.97 

11.97 

"     carbon. 

4 

"    hydrogen. 

Methyl  fluoride     . 

1.186 

34.3 

34.03 

11.97 

"    carbon. 

3 

hydrogen. 

19.06 

"    fluorine. 

GASEOUS  ELEMENTS  AND  COMPOUNDS.         49 


Name. 

d. 

CZX28.& 

M. 

Constituents. 

Methyl  chloride     . 

1.731 

50 

50.34 

11.97  parts  carbon. 

3         "     hydrogen. 

35.37     "     chlorine. 

Methyl  bromide     . 

3.253 

93.9 

94.7S 

11.97      ;     carbon. 

3         "    hydrogen. 

79.76     "    bromine. 

Methyl  iodide  .     . 

4.883 

141 

141.5 

11.97     "    carbon. 

3          "     hydrogen. 

126.54    "    iodine. 

Methyl  nitrate  .     . 

2.64 

76.2 

76.8 

11.97     "     carbon. 

3               hydrogen. 

14.01     "    nitrogen. 

47.88     "    oxygen. 

Methyl  alcohol  .     . 

1.12 

32.3 

31.93 

11.97     "     carbon. 

4         "     hydrogen. 

15.96     "    oxygen. 

Carbon  monoxide  . 

0.968 

27.96 

27.93 

11.97     "    carbon. 

15.96     "    oxygen. 

Carbon  dioxide  .     . 

1.529 

44.16 

43.89 

11.97     "    carbon. 

31.92      '    oxygen. 

Chloroform  .     .     . 

4.2 

121.3 

121.08 

11.97      '    carbon. 

3          '    hydrogen. 

106.11      '     chlorine. 

Carbon  tetrachloride 

5.24 

151.3 

153.45 

11.97      <    carbon. 

141.48      '    chlorine. 

Carbon  oxychloride 

3.505 

101.2 

98.67 

11.97      '    carbon. 

15.96      '    oxygen. 

70.74      '    chlorine. 

Carbon  oxy  sulphide 

2.105 

60.8 

59.91 

11.97      '    carbon 

15.96          oxygen. 

31.98          sulphur. 

Carbon  sulphide     . 

2.645 

76.39 

75.93 

11.97          carbon. 

63.96          sulphur. 

Hydrocyanic  acid  . 

0.948 

27.4 

26.98 

1       p  rt  hydrogen. 

11.97  parts  carbon. 

Cyanogen  chloride 

2.13 

61.5 

61.35 

14.01          nitrogen. 
11.97          carbon. 

14.01           nitrogen. 

35  37          chlorine. 

Cyanic  acid  .     .     . 

1.5 

43.3 

42.94 

1197          carbon. 

14.01     "     nitrogen. 

15.96     "     oxygen. 

Hydrochloric  acid  . 

1.247 

36 

36.37 

1       part  hydrogen. 
35  37  parts  chlorine. 

Chromic    oxychlo- 

1      part  hydrogen. 

ride  

5.55 

160.3 

155.11 

52.45  parts  chromium. 

31.92     "    oxygen. 

70.74    "    chlorine. 

50      PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 


Name. 

d. 

(ZX28.88 

M. 

Constituents. 

Cuprous  chloride   . 

6.93 

200.1 

197.10 

126.36  parts  copper. 

7074     "    chlorine. 

Indium  chloride     . 

7.87 

227.3 

219.51 

113.4      «    indium. 

106.11     "    chlorine. 

Hydriodic  acid  .     . 

4.443 

128.3 

127.54 

126.54    "     iodine. 

1       part  hydrogen. 

Ferric  chloride  .     . 

11.07 

320 

323.98 

111.76  parts  iron. 

212.22     "    chlorine. 

Lead  chloride    .     . 

9.5 

274.4 

277.14 

2064       <    lead. 

70.74     '    chlorine. 

Lead  methyl     .     . 

9.6 

277.2 

266.28 

206.4       '    lead. 

47.88      '     carbon. 

12           <     hydrogen. 

Mercuric  chloride  . 

9.8 

283 

270.54 

199.8        '    mercury. 

70.74     '    chlorine. 

Mercuric  bromide  . 

12.16 

351.2 

359.32 

199.8        '    mercury. 

159.52     '    bromine. 

Mercuric  iodide 

16.2 

468 

452.88 

199.8        '     mercury. 

253.08      '    iodine. 

Mercury  methyl    . 

8.29 

239.4 

229.74 

199.8        '    mercury. 

23.94     '    carbon. 

6          '    hydrogen. 

Mercury  ethyl  .     . 

9.97 

287.9 

367.68 

199.8       '     mercury. 

47.88      '    carbon. 

10           '    hydrogen. 

Molybdic  chloride 

9.46 

273 

272.75 

95.9       '    molybdenum 

176.85     "    chlorine. 

Niobic  chloride 

9.6 

277.2 

270.55 

93.7      "    niobium. 

176.85     "    chlorine. 

Niobic  oxychloride 

7.88 

227.6 

215.77 

93.7       "    niobium. 

15.96     "    oxygen. 

10611     "     chlorine. 

Ammonia     .     .     . 

0.597 

17.2 

17.01 

14.01     "    nitrogen. 

3          "    hydrogen. 

Nitric  oxide  .     .     . 

1.039 

30 

29.97 

14.01     "     nitrogen. 

15.96     "     oxygen. 

Nitrous  oxide    .     . 

1.520 

43.9 

43.98 

28.02     "    nitrogen. 

15.96     "    oxygen. 

Osmium  tetroxide  . 

8.89 

256.7 

258.84 

195         "    osmium. 

63.84     "     oxygen. 

Water.          .     .     . 

0.623 

17.99 

17.96 

15.97     ll     oxygen. 

2          "     hydrogen. 

Phosphine    .     .     . 

1.18 

34.1 

33.96 

30.96     "    phosphorus. 
3          "     hydrogen. 

Phosphorus  trichlo- 

ride      .... 

4.85 

140 

137.07 

30.96     "    phosphorus. 

106.11     "    chlorine. 

GASEOUS  ELEMENTS  AND  COMPOUNDS. 


51 


Name. 

d. 

(JX28.88 

M. 

Constituents. 

Phosphorus      oxy- 

chloride     .     .     . 

5.40 

155.9 

153.03 

30.96  parts  phosphorus. 

15.96 

oxygen. 

106.11 

'*     chlorine. 

Phosphorus  sulpho- 

chloride     .     .     . 

5.88 

169.8 

169.05 

30.96 

"    phosphorus 

31.98 

"     sulphur. 

106.11 

"    chlorine. 

Triethylphosphine 
oxide    .... 

4.60 

132.8 

133.74 

30.96 

"     phosphorus. 

15.96 

"     oxygen. 

71.82 

"     carbon 

15 

"    hydrogen. 

Phosphorus  penta- 

sulphide    .     .     .       7.65 

220.9 

221.82 

61.92 

"    phosphorus. 

159.90 

"    sulphur. 

Selenium  dioxide  . 

4.03 

116.4    110.79!  78.87 

"    selenium. 

31.92 

"     oxygen. 

Silicic  chloride  . 

5.94 

171.5    169.481  28 

"     silicon. 

141.48 

"    chlorine. 

Silicic  fluoride  .     . 

3.6 

104 

104.241  28 

"    silicon. 

7624 

"     fluorine. 

Silicic  iodide     .     . 

19.1 

551.6 

534.14 

28 

"     silicon. 

506.14 

"    iodine. 

Silicon  ethyl      .    \ 

5.14 

148.4 

143.76 

28 

"     silicon 

95.76 

'*    carbon 

20 

"     hydrogen. 

Sulphur  dioxide    . 

2.247 

64.9 

63.9 

31.98 

1     sulphur. 

Sulphur  trioxide    . 

3.01 

86.9 

79.76 

31.92 
31.98 

'    oxygen. 
:     sulphur. 

47.88 

"     oxygen 

Sulphuryl  chloride 

4.67 

134.8 

134.64 

31.98 

i(    sulphur. 

31.92 

"     oxygen. 

70.74 

"     chlorine. 

Hydrogen  sulphide 

1.191 

34.4 

33.98 

3198 

"     sulphur. 

2 

"     hydrogen. 

Tantalic  chloride  . 

12.9 

372.5 

358.85 

182 

"     tantalum. 

176.85 

"     chlorine. 

Thallic  chloride     .       8.15 

235.4 

239.07 

203.7 

"     thallium. 

35.37 

"    chlorine. 

Stannous  chloride  . 

12.96 

3743 

375.28 

234.8 

"    tin. 

141.48 

"     chlorine. 

Stannic  chloride     . 

9.20 

265.7 

258.88 

117.4 

"    tin. 

141.48 

il    chlorine 

Tin  ethyl,     .     .     . 

8.02 

231.6 

233.16 

117.4 

"    tin 

95.76 

"    carbon. 

20 

"     hydrogen. 

52       PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 


Name. 

d. 

dX28.88 

M. 

Constituents. 

Tin  triethyl  chloride 

8.43 

243.4 

239.59 

117.4 

parts  tin. 

35.37 

'     chlorine. 

71.82 

"    carbon. 

Tin  triethyl  bromide 

9.92 

286.5 

283.98 

15 
117.4 

"     hydrogen. 
"     tin. 

79.76 

"    bromine. 

71.82 

"    carbon. 

15 

"     hydrogen. 

Titanic  chloride     . 

6.84 

197.5 

189.48 

48 

"    titanium. 

141.48 

"    chlorine. 

Tungsten  pentachlo- 

ride  

12.7 

366.7 

360.45 

183.6 

^     tuncrstcn.* 

176*85 

"    chlorine. 

Tungsten  hexachlo- 

ride  

13.2 

381.2 

395.82 

183.6 

"     tungsten. 

21  2.22 

"     chlorine. 

Tungsten  oxychlo- 

ride  

11.84 

342 

341.04 

183.6 

"     tungsten. 

15.96 

"    oxygen. 

141.48 

"    chlorine. 

Uranium  chloride  . 

13.3 

384.1 

381.28 

239.8 

"    uranium. 

141.48 

"    chlorine. 

Uranium  bromide  . 

19.46 

562 

558.84 

239.8 

"     uranium. 

319.04 

"     bromine. 

Vanadium  tetrachlo- 

ride       •     •     •     • 

6.78 

195.8 

192.58 

51.1 

11     vanadium. 

141.48 

"    chlorine. 

Vanadium  oxy  chlo- 

ride .     .     .     . 

6.11 

176 

173.17 

51.1 

"     vanadium. 

15.96 

'(     oxygen. 

106.11 

"     chlorine. 

Zinc  chloride     .     . 

4.57 

132 

135.62 

64.88 

"    zinc. 

70.74 

"    chlorine. 

Zinc  methyl  .     .     „ 

3.29 

95.1 

94.82 

64.88 

4<     zinc. 

23.94 

"     carbon. 

6 

"    hydrogen. 

Zinc  ethyl     .     .     „ 

4.62 

133 

122.76 

64.88 

"     zinc. 

47.88 

'*    carbon. 

Zinc  amyl     .     .     . 

6.95 

200.7 

206.58 

10 

64.88 

"    hydrogen. 
"    zinc. 

119.70 

"     carbon. 

Zirconium  chloride 

8.15 

235.4 

232.42 

22 

90.94 

"     hydrogen. 
"    zirconium 

141.48 

"    chlorine. 

GASEOUS  ELEMENTS  AND  COMPOUNDS.         53 

This  list  contains  compounds  of  thirty  elements,  and,  by 
means  of  these  compounds,  assuming  that  they  contain  at 
least  one  of  their  elements  in  the  smallest  possible  propor- 
tion, we  can  determine  the  atomic  weights  of  the  elements 
concerned.  It  will  be  seen,  however,  that  while  the  hy- 
pothesis of  Avogadro  furnishes  a  method  that  enables  us  to 
state  positively  what  the  molecular  weight  of  any  gaseous 
compound  is,  it  does  not  furnish  a  means  for  the  determi- 
nation of  atomic  weights  directly.  After  examining  the 
various  compounds  of  an  element,  that  one  is  selected 
which  contains  the  smallest  quantity  of  the  element  in  the 
molecule,  and  then,  without  further  proof,  we  say  this 
smallest  quantity  represents  the  atom.  Thus  it  is  evident 
that  the  atomic  weights,  as  determined  by  this  method,  rest 
upon  a  more  or  less  doubtful  basis.  For  practical  pur- 
poses, however,  this  is  not  a  serious  matter ;  inasmuch  as, 
although  we  cannot  assert  that  the  weight  found  really 
represents  the  atomic  weight,  we  can  assert  that  it  repre- 
sents the  weight  of  that  portion  of  the  element  which  con- 
ducts itself  as  an  atom — i.  e.,  throughout  the  series  of 
changes  which  it  undergoes  in  its  compounds  it  is  indi- 
visible. 

Other  Proofs  of  the  Fact  that  the  Molecules  of  Elements 
contain  more  than  one  Atom. — It  has  been  stated  above  that 
in  most  cases  the  molecules  of  elements  contain  more  than 
one  atom ;  and  it  has  been  shown  that,  if  the  hypothesis  of 
Avogadro  is  accepted,  we  are  necessarily  led  to  this  con- 
clusion by  a  simple  consideration  of  the  molecular  weights 
of  elements  and  their  compounds.  The  question  will  natu- 
rally suggest  itself:  Is  there  any  evidence  independent  of 
Avogadro's  hypothesis  that  the  molecules  of  elements  con- 
sist of  more  than  one  atom  ?  There  are  at  least  some  indi- 
cations that  this  is  the  fact. 

A  number  of  the  elements,  as  we  ordinarily  meet  with 
them  in  the  free  state,  conduct  themselves  as  comparatively 
inactive  substances.  Take,  for  instance,  hydrogen.  In  its 
usual  condition,  this  element  does  not  act  readily  upon  most 
other  elements  and  compounds.  It  can  be  brought  in  di- 
rect contact  with  most  substances  without  effecting  any 
change  in  them.  If,  however,  it  is  set  free  from  one  of  its 
compounds,  and,  at  the  instant  it  is  set  free,  it  is  allowed  to 
act  upon  some  other  substance,  it  is  found  to  be  a  compara^ 


54       PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

tively  active  substance,  capable  of  effecting  material  changes 
in  other  substances.  The  same  is  true  of  oxygen,  nitrogen, 
and  other  elements.  They  are  much  more  active  in  the 
nascent  state  than  in  the  free  state.  How  can  this  be  ex- 
plained ?  Most  readily  by  supposing  the  molecules  of  these 
elements,  in  the  free  state,  to  contain  more  than  one  atom 
combined  with  each  other.  Now,  if  it  is  required  that  an 
element  thus  constituted  shall  combine  with  another  sub- 
stance, it  is  first  necessary  that  the  force  which  holds 
together  the  atoms  be  overcome ;  the  atoms  must  be  torn 
asunder  before  they  can  act  as  atoms ;  or,  in  other  words, 
a  decomposition  must  be  effected  before  the  required  com- 
bination can  take  place.  Sometimes  the  action  exerted  by 
an  atom  of  one  element  on  an  atom  of  another  is  so  strong 
that  this  decomposition  is  effected,  and  the  combination 
then  takes  place.  Thus,  if  we  bring  hydrogen  and  chlorine 
together,  both  in  the  free  state,  they  combine.  In  this  case, 
the  chlorine  atom  unites  with  the  hydrogen  atom  more  readily 
than  the  hydrogen  atom  unites  with  another  hydrogen  atom, 
or  the  chlorine  atom  with  another  chlorine  atom.  On  the 
other  hand,  numerous  instances  of  the  opposite  kind  might 
be  adduced.  One  will  suffice  for  the  purpose  of  illustra- 
tion. When  hydrogen  gas  is  passed  through  nitric  acid  no 
change  takes  place ;  but  when  zinc  is  dissolved  in  nitric 
acid  a  portion  of  the  acid  is  decomposed  by  the  hydrogen 
evolved.  The  hydrogen  atoms,  set  free  from  the  nitric 
acid,  find  the  acid  present,  and  act  upon  it  in  preference  to 
combining  to  form  free  hydrogen  ;  the  elements  in  combi- 
nation with  nitrogen  are  forcibly  removed  and  the  hydro- 
gen takes  their  place,  forming  ammonia. 

Ordinary  oxygen  contains  two  atoms  in  the  molecule. 
Ozone,  another  variety  of  oxygen,  has  the  density  1.658, 
from  which  its  molecular  weight  is  found  to  be  nearly  48. 
Now  as  the  atomic  weight  of  oxygen,  according  to  previous 
determinations,  is  15.96,  it  follows  that  the  molecule  of 
ozone  contains  three  atoms.  The  difference  between  the 
two  forms  of  oxygen  is  thus  explained.  Ozone  is  compar- 
atively unstable.  It  gives  up  its  extra  atom  with  great 
ease  to  substances  with  which  it  comes  in  contact,  and  thus 
causes  energetic  oxidation.  When  heated  to  300°,  it  is 
decomposed,  forming  ordinary  oxygen,  and  then  an  increase 
of  volume  is  observed.  In  this  case,  if  no  foreign  substance 
is  present  with  which  the  liberated  atom  can  unite,  it  unites 


GASEOUS  ELEMENTS  AND  COMPOUNDS.         55 

with  another  atom  of  the  same  kind.  When  it  acts  upon 
other  substances  the  original  volume  of  the  gas  remains 
unchanged.  It  thus  appears  that  the  two  atoms  of  the 
molecule  of  ordinary  oxygen  are  held  together  more  firmly 
than  the  three  atoms  in  the  molecule  of  ozone.  Here 
again  the  different  actions  of  the  two  varieties  can  appar- 
ently be  best  explained  by  supposing  in  each  case  the  mole- 
cule to  consist  of  more  than  one  atom. 

These  and  similar  considerations  serve  to  strengthen  the 
conclusion  to  which  we  are  led  by  accepting  Avogadro's 
hypothesis,  and  hence,  in  turn,  to  increase  the  probability 
that  the  hypothesis  is  in  fact  a  law. 

Molecular  Formulas  of  Gaseous  Compounds. — When  the 
atomic  weights  of  the  elements  are  once  determined  the 
law  of  Avogadro,  taken  together  with  chemical  analysis, 
is  sufficient  to  enable  us  to  determine  the  molecular  for- 
mulas of  gaseous  chemical  compounds — a  problem,  the 
solution  of  which  without  this  rule  is  in  many  cases  ex- 
ceedingly difficult,  and,  indeed,  at  times  impossible.  Let 
us  suppose  the  atomic  weights  of  carbon  (11.97),  hydro- 
gen (1),  and  oxygen  (15.96)  to  be  known.  We  analyze 
a  certain  compound,  and  find  that  it  contains  37.49  per 
cent,  carbon,  12.53  per  cent,  hydrogen,  and  49.98  per  cent, 
oxygen.  This  gives  the  atomic  proportion  1  of  carbon,  4 
of  hydrogen,  and  1  of  oxgen ;  and  hence  the  simplest 
formula  that  can  be  assigned  to  the  compound  is  CH4O. 
But  the  formulas  C2H8O2  or  C3H12O4,  etc.,  also  satisfy  the 
results  of  the  analysis.  The  molecular  weight  is  next  de- 
termined by  Avogadro's  method,  and  it  is  found  to  be 
approximately  32 ;  and,  as  the  sum  of  the  weights  of  the 
atoms  in  a  molecule  of  a  compound  of  the  formula  CH4O 
is  31.93,  this  formula  is  thus  shown  to  be  the  correct  one. 

Apparent  Exceptions. — All  formulas  of  chemical  com- 
pounds at  present  in  use  are  intended  to  represent  mole- 
cules of  the  compounds.  They  are  molecular  formulas. 
They  represent  those  quantities  of.  the  compounds  which, 
in  a  gaseous  condition,  would  occupy  the  same  space  as  a 
molecule  of  hydrogen.  If  we  take  two  volumes  of  hydrogen 
as  the  standard  of  comparison,  then  the  formulas  of  com- 
pounds represent  two  volumes  of  the  same  size.  To  this 
there  are  apparent  exceptions.  When  ammonia  acts  upon 
hydrochloric  acid  the  two  gases  combine  in  the  proportion 


56       PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

of  1  vol.  of  the  one  to  1  vol.  of  the  other,  forming  a  solid 
compound  which  contains  26.25  per  cent,  nitrogen,  7.49 
per  cent,  hydrogen,  and  66.36  per  cent,  chlorine.  This 
gives  the  atomic  proportion  1  nitrogen,  4  hydrogen,  and  1 
chlorine ;  and  the  simplest  formula  that  can  be  assigned 
to  the  compound,  provided  the  atomic  weights  of  nitrogen, 
hydrogen,  and  chlorine  are  respectively  14.01, 1,  and  35.37, 
is  NH4C1.  On  now  determining  the  molecular  weight  by 
Avogadro's  method,  this  is  found  to  be  26.69,  or  half  that 
required  by  the  above  formula.  Evidently,  it  is  impossible 
for  a  molecule  made  up  of  chlorine,  nitrogen,  and  hydro- 
gen, with  the  atomic  weights  above  assigned  to  them,  to 
have  as  small  a  weight  as  26.69;  and,  to  satisfy  the  results 
of  this  determination,  we  should  be  obliged  to  write  the 
formula  NHH2C1M,  and  thus  accept  the  existence  of  half 
atoms,  which  is  absurd.  We  might  also  imagine  the  atomic 
weight  of  nitrogen  and  chlorine,  as  already  determined,  to 
be  just  twice  too  great;  for  if  we  assign  to  nitrogen  the 
atomic  weight  7,  and  to  chlorine  17.69,  we  could  write  the 
formula  of  the  compound  NH2C1,  and  this  compound  would 
have  the  molecular  weight  26.69,  as  determined  by  Avo- 
gadro's method.  On  the  other  hand,  if  7  is  the  atomic 
weight  of  nitrogen,  aod  17.69  that  of  chlorine,  then  in  all 
other  compounds  of  nitrogen  or  chlorine,  in  which  one 
atom  has  been  supposed  to  exist  in  the  molecule,  we  must 
necessarily  accept  the  existence  of  two  atoms  in  the  mole- 
cule. But  then  all  these  compounds  would  not  come 
under  Avogadro's  rule.  Hence  we  see  that  the  compound 
NH4C1  is  clearly  an  exception,  and,  if  no  satisfactory  ex- 
planation can  be  found  to  account  for  this  case,  its  exist- 
ence is  fatal  to  the  rule.  A  satisfactory  explanation  can 
be  offered,  however,  as  follows : 

If  it  can  be  proved  that  the  vapor  obtained  from  the 
compound  NH4C1  is  not  the  vapor  of  this  compound,  but 
a  mixture  of  the  vapors  of  its  constituents,  ammonia,  NH3, 
and  hydrochloric  acid,  HC1,  the  case  becomes  a  very  simple 
one.  Without  entering  into  details,  it  may  be  stated 
that  the  results  of  the  experiments  made  upon  this  sub- 
ject have  justified  the  assumption  that,  when  the  com- 
pound NH4C1  is  heated  to  a  temperature  sufficiently  high 
to  convert  it  into  vapor,  it  is  broken  down  into  ammonia, 
NH,,  and  hydrochloric  acid,  HC1,  and  that,  when  this 


GASEOUS  ELEMENTS  AND  COMPOUNDS.         57 

mixture  is  cooled  down,  the  two  compounds  again  unite  to 
form  the  original  compound. 

As  ammonia,  NH3,  and  hydrochloric  acid,  HC1,  combine 
volume  to  volume,  so  the  mixture  of  the  two  gases,  ob- 
tained by  heating  ammonium  chloride,  NH4C1,  consists  of 
equal  volumes  of  the  two  ;  and  the  specific  gravity  of  this 
mixed  vapor  should  be  the  mean  of  the  specific  gravities 
of  its  constituents.  The  specific  gravity  of  ammonia  is 
0.597,  that  of  hydrochloric  acid  is  1.247 ;  the  specific  gravity 

of  a  mixture  of  the  two  would  be  -  —  —  0.922 ; 

2> 

and  this  specific  gravity,  if  it  is  that  of  a  chemical  com- 
pound, leads  to  the  molecular  weight  26.6.  The  number 
0.922  is,  indeed,  that  found  as  the  specific  gravity  of  the 
vapor  of  ammonium  chloride,  and  it  will  thus  be  seen  that 
the  fact  can  be  satisfactorily  explained  without  giving  up 
our  belief  in  Avogadro's  law.  Further,  Neuberg  has 
shown  that  when  ammonium  chloride  is  vaporized  in  the 
presence  of  ammonia  or  of  hydrochloric  acid  the  specific 
gravity  approximates  that  required  by  the  theory  for  a 
compound  of  the  formula  NH4C1. 

The  compound  phosphorus  pentachloride,  PCJ5,  was  also 
at  one  time  regarded  as  an  exception  to  the  rule.  The  spe- 
cific gravity  of  its  vapor  was  found  to  be  3.66,  from  which 
the  molecular  weight  105  7  was  calculated,  half  that  re- 
quired by  the  formula  PC!6.  It  has  been  shown,  however, 
that  above  a  certain  temperature  the  molecule  of  this  com- 
pound breaks  down  into  a  molecule  cf  phosphorus  trichlo- 
ride, PCJ3,  and  a  molecule  of  chlorine,  C12.  The  vapor 
from  phosphorus  pentachloride,  PC15,  is  a  mixture  of  two 
vapors,  and  the  mean  specific  gravity  of  the  two  is  the 
specific  gravity  found.  The  specific  gravity  of  the  vapor 
of  the  trichloride,  PC13,  is  4.88 ;  that  of  chlorine  is  2.45 ; 

the  mean  specific   gravity  -  —  =  3.666.     In  this 

tt 

case  it  has  been  shown  further  that  the  breaking  down  of 
the  molecule  takes  place  gradually ;  for  the  specific  gravity 
of  the  vapor  decreases  from  5.08  to  3.66,  as  the  tempera- 
ture is  raised  from  182°  to  300°,  at  which  latter  tempera- 
ture the  decomposition  appears  to  be  complete,  no  further 
decrease  in  specific  gravity  being  observed  when  the  vapor 
is  heated  still  higher. 


58       PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

It  has  also  been  shown  that  when  the  specific  gravity 
of  the  vapor  of  phosphorus  pentachloride,  PC15,  is  deter- 
mined in  the  presence  of  the  trichloride,  PCI,,  the  de- 
composition does  not  take  place,  and  the  specific  gravity 
found  is  that  which  theory  requires  for  a  compound  of  the 
formula  PC15.  The  mean  result  of  7  determinations  was 
7,226,  whereas,  the  theory  requires  7.217.  From  this  it  is 
evident  that  the  hypothesis  of  Avogadro  is  valid  for  the 
compound  PC15,  as  well  as  for  other  compounds. 

A  third  example  of  this  kind  of  decomposition  by  increase 
of  temperature  is  met  with  in  the  case  of  the  compound 
NO2.  This  compound,  at  a  low  temperature,  consists  of 
colorless  crystals.  At  a  slightly  elevated  temperature 
these  crystals  change  to  a  yellow  liquid.  The  liquid  boils 
at  20°-30°,  and  is  then  converted  into  a  gas  of  a  reddish  - 
brown  color,  and  as  the  temperature  of  the  gas  is  raised 
the  intensity  of  the  color  increases.  Above  500°  the  color 
begins  to  grow  weaker  on  account  of  the  decomposition  of 
the  oxide  NO2  into  oxygen  and  nitric  oxide,  2NO2  =  2NO 
-|-  O2.  The  specific  gravity  of  the  gas  decreases  with  this 
elevation  of  temperature;  hence,  it  is  supposed  that  the 
compound,  at  a  low  temperature,  is  properly  represented 
by  the  formula  N2O4,  but  that  the  molecule  is  broken  down 
by  heat,  two  molecules  of  NO2  being  formed.  The  latter 
is  strongly  colored,  and  the  more  of  it  there  is  present  in 
the  mixture,  the  more  intense  is  the  color  of  the  gas. 

Among  chemical  compounds  there  are  few  that  conduct 
themselves  like  the  three  just  described.  As  regards  some 
of  these,  good  proof  can  be  given  that  their  molecules  are 
broken  down  by  conversion  into  vapor,  and,  hence,  the 
apparently  abnormal  specific  gravities  observed  for  these 
vapors  find  a  simple  explanation.  As  regards  others, 
although  positive  proof  to  the  same  effect  may  indeed  be 
lacking  as  yet,  still  strong  indications  are  presented  that 
the  abnormal  densities  are  due  to  the  same  cause.  So 
that,  up  to  the  present,  not  only  is  no  fact  known  that 
speaks  strongly  against  Avogadro's  hypothesis,  but,  on  the 
contrary,  new  developments  are  constantly  tending  to 
strengthen  it.  It  forms,  to-day,  by  far  the  most  reliable 
means  for  the  determination  of  molecular  weights  of  com- 
pounds and  elements ;  and  we  have  seen  how,  indirectly, 
it  aids  us  in  determining  atomic  weights.  But,  in  order 
that  it  may  be  useful,  it  is  necessary  that  the  compound 


GASEOUS  ELEMENTS  AND  COMPOUNDS.         59 

which  we  desire  to  study  shall  be  capable  of  conversion 
into  vapor,  or,  if  an  element  is  under  consideration,  that 
at  least  one  of  the  compounds  of  this  element  be  gaseous 
or  volatile.  Only  a  comparatively  small  number  of  com- 
pounds satisfy  these  conditions,  and  of  the  70  elements, 
only  36  (see  List,  pp.  48-52)  enter  into  the  composition 
of  these  compounds.  With  no  other  means,  then,  at  our 
command,  the  work  would  be  incomplete.  It  is  desirable 
that  some  other  method  should  be  introduced  which  shall 
be  applicable  to  those  elements  not  covered  by  Avogadro's 
rule — i.  e.,  those  elements  which  are  themselves  incapable 
of  conversion  into  vapor,  and  which  do  not  enter  into  the 
composition  of  gaseous  or  volatile  compounds. 


60       PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 


CHAPTER  IV. 

A   STUDY   OF   SOLUTIONS. 

THE  phenomena  of  solution  have  long  been  the  subject 
of  study,  but  not  until  the  last  few  years  have  results  been 
reached  of  importance  in  connection  with  the  problem  of 
the  determination  of  molecular  weights.  Through  the 
labors  of  Raoult,  Pfeffer,  Van't  Hoff,  and  others,  relations 
have  been  discovered  between  the  freezing-point,  the  boil- 
ing-point, and  the  osmotic  pressure  of  solutions  on  the  one 
hand,  and  the  molecular  weights  of  the  dissolved  substances 
on  the  other. 

Relation  between  the  Vapor-pressure  of  Solutions  and  the 
Molecular  Weights  of  the  dissolved  Substances.  —  Although 
some  generalizations  were  established  by  the  study  of  water 
solutions,  it  was  not  until  Raoult  (1886)  investigated  ether 
solutions,  that  the  connection  between  the  vapor-pressure 
of  solutions  and  the  molecular  weights  of  the  dissolved 
substances  was  discovered.  The  principal  conclusion  to  be 
drawn  from  these  investigations  may  be  stated  thus  :  If 
n  =  the  number  of  molecules  of  the  dissolved  substance, 
JV=  the  number  of  molecules  of  the  solvent,  p  —  vapor- 
pressure  of  pure  ether,  p'  =  vapor-pressure  of  an  ethereal 
solution,  then 


p  N+n 

where  c  is  a  constant  which  falls  between  .96  and  1.09. 

The  law  may  also  be  stated  thus  :  — 

The  relative   lowering  of  vapor  pressure  is  propor- 

tional to  the  ratio  of  the  number  of  molecules  of  the  dis- 

solved substance  to  the  total  number  of  molecules  in  the 

solution. 

The  relation  expressed   by  this  law  plainly  furnishes  a 

new  method  for  the  determination  of  molecular  weights. 

In  this  case  it  is  not  necessary  that  a  substance  should  be 

capable  of  conversion  into  vapor  ;  it  is  only  necessary  that 


A  STUDY  VF  SOL  UTIONS.  6  1 

it  should  be  soluble  in  a  solvent  for  which  the  law  has  been 
found  to  hold  good. 

In  practice  it  has  been  found  most  convenient  to  make 
observations  on  the  boiling-points  of  solutions,  and  a  method 
has  been  devised  based  upon  such  observations.  Ether  is 
most  frequently  used  as  the  solvent. 

Relation  between  the  Freezing-points  of  Solutions  and  the 
Molecular  Weights  of  the  dissolved  Substances.  —  The  researches 
of  Raoult  on  the  freezing-points  of  solutions  led  him  to  the 
discovery  of  the  following  law  :  — 

"  One  molecule  of  any  compound,  when  dissolved  in 
100  molecules  of  a  liquid,  lowers  the  freezing-point  of  the 
liquid  by  an  amount  which  is  nearly  constant,  viz.,  0.62°." 
Let  P  =  weight  of  substance  dissolved  ; 
L  —  weight  of  solvent  ; 
M  =  molecular  weight  of  solvent  ; 
E  =  lowering  of  freezing-point  ; 
m  =  molecular  weight  of  the  dissolved  substance. 
Then 


or, 


LxE 

For  most  so-called  inorganic  salts  this  law  does  not  hold 

ood,  as  will  be  pointed  out  more  fully  below  ;  but  it  does 

old  for  many  classes  of  organic  compounds  and  organic 

solvents.     Glacial  acetic  acid  and  benzene  are  most  com- 

monly used  as  solvents. 

The  method  of  Raoult  for  the  determination  of  molecular 
weights,  based  upon  observations  on  the  freezing-points 
of  solutions,  has  come  into  extensive  use  since  the  year 
1888. 

Relation  between  the  Osmotic  Pressure  of  Solutions  and  the 
Molecular  Weights  of  the  dissolved  Substances.  —  During  the 
last  century  it  was  observed  that,  if  a  vessel  is  filled  with 
alcohol  and  the  vessel  tightly  covered  by  tying  a  bladder 
over  its  mouth,  and  the  whole  then  immersed  in  water,  the 
bladder  is  stretched  outward,  showing  that  liquid  from 
without  has  found  its  way  within  the  vessel  through  the 
bladder.  For  many  years  investigations  on  phenomena  of 
this  kind  were  carried  on,  but  no  results  of  a  general  char- 

4 


62      PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

acter  of  special  importance  from  a  chemical  point  of  view 
were  reached  until  recently.  In  a  book  entitled  Osmotische 
Untersuchungen,  published  in  1877,  Pfeffer,  the  botanist, 
gave  an  account  of  a  large  number  of  experiments,  and 
laid  the  foundation  of  our  present  knowledge  of  the  laws 
of  osmotic  phenomena.  The  simplest  phenomenon  of  this 
kind  is  seen  when  a  wide  glass  tube,  tightly  closed  with  a 
piece  of  bladder  at  the  lower  end,  is  partly  filled  with  alco- 
hol and  then  placed  in  a  vessel  of  water,  so  that  the  level 
of  the  liquids  inside  and  outside  the  tube  is  the  same.  If 
the  tube  is  fixed  in  this  position,  the  liquid  is  soon  found  to 
rise  in  the  tube.  There  is  pressure  from  without  inward. 
This  is  called  osmotic  pressure.  PfefFer's  investigations  had 
to  do  with  the  measurement  of  the  pressure  exerted  by 
different  substances  under  different  conditions  of  tempera- 
ture and  dilution.  Instead  of  using  a  bladder,  which  is 
not  capabje  of  much  resistance,  he  used  membranes  made 
by  precipitating  copper  ferrocyanide  in  the  pores  of  clay- 
cells.  Somewhat  later,  Van't  Hoff  showed  that  the  results 
obtained  by  Pfeffer  led  to  the  following  remarkable  laws 
governing  osmotic  pressure: — 

1.  The  pressure  is  proportional  to  the  concentration, 
or  it  is  inversely  proportional  to  the  volume  in  which  a 
definite  quantity  of  the  dissolved  substance  is  contained. 

2.  The  pressure  increases,  for  constant  volume,  pro- 
portionally to  the  absolute  temperature. 

3.  Quantities  of  dissolved  substances  which  are  in  the 
ratio  of  the  molecular  weights  of  these  substances  exert 
equal  pressure  at  equal  temperatures. 

These  laws  are  analogous  to  the  well-known  laws  of  gases, 
the  third  being  plainly  analogous  to  the  law  of  Avogadro. 
Another  form  of  stating  the  third  law,  together  with  an 
extension  of  it,  is  this : — 

Dissolved  substances  exert  the  same  pressure,  in  the 
form  of  osmotic  pressure,  as  they  would  exert  were  they 
gasified,  at  the  same  temperature,  without  change  of 
volume. 

Notwithstanding  the  simplicity  of  this  law,  no  practical 
method  of  determining  molecular  weights  based  upon  it  has 
yet  been  devised.  The  difficulties  are,  however,  mostly  of 
an  experimental  nature. 

Since  Pfeffer's  observations  were  published,  other  meth- 
ods of  determining  osmotic  pressure  have  been  worked  out, 


A  STUDY  OF 

and  the  results  obtained  have  been  found  to  agree  with  those 
obtained  by  Pfeffer.  Prominent  among  these  methods  is 
that  of  de  Vries.  It  depends  upon  observations  on  organic 
cells  in  solutions  of  different  concentrations.  When  such 
a  cell  containing  protoplasm  is  introduced  into  a  solution 
it  will  change  its  form  perceptibly  unless  the  osmotic  pres- 
sure of  the  solution  is  the  same  as  that  of  the  contents  of 
the  cell.  By  changing  the  concentration  of  the  solution  it 
can  be  brought  to  such  a  condition  that  the  cell  retains  its 
original  form.  By  this  means  it  is  possible  to  compare  dif- 
ferent solutions  and  to  determine  the  concentration  of  the 
solutions  that  exert  the  same  osmotic  pressure. 

Exceptions  to  the  Laws  of  Solutions. — The  laws  governing 
the  relations  between  the  molecular  weights  of  a  dissolved 
substance  and  the  freezing-point,  boiling-point,  and  osmo- 
tic pressure  of  the  solution  are  found  not  to  hold  good  for 
water  solutions  of  salts,  strong  acids,  and  bases.  Such  solu- 
tions act  as  if  they  contained  a  larger  number  of  molecules 
than  is  indicated  by  their  formulas.  Thus  in  a  dilute  solu- 
tion of  sodium  chloride  there  appear  to  be  two  molecules 
for  every  molecule  of  the  substance  added.  To  account  for 
this,  it  has  been  suggested  by  Arrhenius  that,  in  these  cases, 
the  dissolved  substances  are  separated  into  their  ions  by  the 
action  of  the  solvent.  A  similar  suggestion  had  been  pre- 
viously made  by  Williamson  and  by  Clausius  for  entirely 
different  reasons.  The  solutions  that  give  the  peculiar  re- 
sults, that  is,  the  water  solutions  of  salts,  and  strong  acids 
and  bases,  differ  from  other  solutions  in  this  respect,  that 
they  are  good  conductors  of  the  electric  current,  and  this 
conduction  is  accompanied  by  a  movement  of  the  particles 
of  the  dissolved  substance  which  is  separated  into  its  ions. 
According  to  the  conception  of  Clausius  and  Arrhenius, 
then,  when  a  salt  is  dissolved  in  water  it  is  separated  to 
some  extent  into  its  ions  by  the  action  of  the  water,  the 
greater  the  dilution  the  more  complete  the  separation. 
Under  ordinary  conditions  these  ions  remain  in  the  solution 
highly  charged  with  electricity.  As  they  move  through 
the  solution  they  come  in  contact  with  each  other,  and 
combinations  and  decompositions  are  constantly  taking 
place.  When  such  a  solution  is  made  part  of  an  electric 
circuit  the  electricity  directs  these  ions,  those  charged  with 
positive  electricity  in  one  direction  toward  one  pole,  those 
charged  with  negative  electricity  toward  the  other  pole. 


64      PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

As  soon  as  an  ion  reaches  its  proper  pole  its  electricity  is 
discharged  and  it  is  set  free. 

It  will  be  seen  that  the  hypothesis  of  dissociation  in 
solution  accounts  for  the  results  obtained  in  studying  the 
freezing-points,  etc.,  of  water  solutions,  in  much  the  same 
way  that  the  peculiar  results  obtained  in  studying  the 
vapor- densities  of  ammonium  chloride  and  of  phosphorus 
pentachloride  are  explained  by  the  assumption  that  these 
substances  undergo  dissociation  when  heated. 

The  hypothesis  of  Arrhenius  is  still  under  discussion, 
though  it  has  rapidly  grown  in  favor  during  the  past  few 
years,  and,  apparently,  those  who  have  given  most  time  to 
the  study  of  the  phenomena  to  which  it  applies,  and  who 
therefore  are  best  qualified  to  pass  judgment  upon  it,  are 
the  ones  who  receive  it  most  cordially. 


SOLID  ELEMENTS  AND  COMPOUNDS.  65 


CHAPTER  V. 

EXAMINATION    OF    SOLID    ELEMENTS    AND    COMPOUNDS. 

Specific  Heat. — It  is  known  that,  when  equal  weights  of 
different  substances  are  exposed  to  heat  from  the  same 
source,  they  have  different  temperatures  at  the  end  of  the 
same  period  of  time.  From  this  it  is  clear  that  to  raise 
equal  weights  of  different  substances  through  the  same 
number  of  degrees  of  temperature,  different  quantities  of 
heat  are  necessary.  Given  exactly  the  same  heating-power, 
it  takes  about  32  times  as  long  to  raise  the  temperature  of 
a  pound  of  water  10,  20,  or  30  degrees  as  it  takes  to  raise 
the  temperature  of  a  pound  of  mercury  the  same  number 
of  degrees ;  or  it  takes  32  times  as  much  heat  to  raise  the 
temperature  of  a  pound  of  water  10,  20,  or  30  degrees  as  it 
takes  to  raise  the  temperature  of  a  pound  of  mercury  the 
same  number  of  degrees.  The  quantity  of  heat  required 
to  raise  the  temperature  of  a  given  weight  of  any  substance 
a  given  number  of  degrees,  as  compared  with  the  quantity 
of  heat  required  to  raise  the  temperature  of  the  same 
weight  of  water  the  same  number  of  degrees,  is  called  the 
specific  heat  of  the  substance.  The  quantity  of  heat  re- 
quired to  raise  the  temperature  of  a  pound  of  water  1  de- 
gree Centigrade  may  be  conveniently  adopted  as  the  thermal 
unit.  The  specific  heat  of  water  is  then  =  1 ;  and  the 
specific  heat  of  any  other  substance  is  the  relative  quantity 
of  heat  necessary  to  raise  the  temperature  of  a  pound  of 
this  substance  1  degree  Centigrade,  taking  the  above  ther- 
mal unit  as  the  standard.  The  specific  heat  of  mercury, 
according  to  the  results  of  the  experiment  mentioned,  is 
0.03332;  that  of  gold  is  found  to  be  0.03244,  etc.,  etc. 
The  meaning  of  these  numbers  will  be  readily  understood. 

Relations  between  Specific  Heat  and  Atomic  Weight. — 
Now,  when  the  solid  elements  are  examined  with  reference 
to  their  specific  heats,  a  very  simple  relation  is  found  to 
exist  between  the  numbers  expressing  the  specific  heats 


66       PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

and  the  atomic  weights.     This  relation  will  be  made  clear 
by  the  following  examples : — 

Element.  Specific  heat.         Atomic  weight. 

Silver.  0.0570  107.66 


Zinc    . 
Cadmium 
Copper 
Tin 


0  0955  64  88 

0  0567  111  7 
00952  63.18 

0.0562  117.4 


It  will  be  seen  by  an  examination  of  this  table  that  the 
atomic  weights  are  inversely  proportional  to  the  specific 
heats.  We  have 

107.66  :  64.88  :  :  0.0955  :  0.0570; 
111.7  :  63.18  :  :  0.0952  :  0.0567; 
107.66  :  117.4  :  :  0.0562  :  0.0570,  etc. 

These  proportions  are  only  approximately  correct ;  but 
it  must  be  remembered  that  the  means  for  the  determina- 
tion of  atomic  weights  and  specific  heats  are  not  perfect, 
and  in  both  sets  of  figures  there  are  probably  some  slight 
errors.  Hence,  such  slight  variations  from  absolute  agree- 
ment in  these  proportions  are  not  surprising.  The  agree- 
ment is  sufficiently  close  to  indicate  a  close  connection 
between  the  two  sets  of  numbers.  This  connection  may  be 
stated  in  another  way :  The  product  of  the  atomic  weight 
by  the  specific  heat  is  a  constant  quantity  for  the  elements 
examined.  Thus,  in  the  above  cases  :  — 


107.66  X  0.057  =  6.14 
64.88  X  0.0955  ==  6.20 

111.7  X  0.0567  =  6.33 
6318  X  0.0952  =  601 

117.4    X  0.0562  =  6  51 


For  the  same  weights,  then,  the  quantities  of  heat  neces- 
sary to  elevate  the  temperature  of  the  elements  one  degree 
vary.  The  quantity  necessary  to  elevate  the  temperature 
of  an  atom  one  degree  is,  of  course,  represented  by  the 
variable  quantity  multiplied  by  the  atomic  weight,  and 
this  product,  in  the  cases  cited,  is  a  constant. 

Investigations  of  Dulong  and  Petit. — In  the  year  1819 
attention  was  first  called  to  the  above  relation  by  Dulong 
and  Petit,  and  having  examined  a  large  number  of  ele- 
ments they  felt  justified  in  propounding  the  law  :  The  atoms 


SOLID  ELEMENTS  AND  COMPOUNDS.  67 

of  all  elements  have  the  same  capacity  for  heat.  This  is  sim- 
ply a  generalization  from  the  facts  stated,  and  is  another 
way  of  stating  that,  to  raise  the  temperature  of  an  atom 
one  degree,  the  same  quantity  of  heat  is  always  necessary. 

If  the  law  propounded  is  in  reality  a  law,  it  will  readily 
be  seen  that  a  new  method  is  given  for  the  determination 
of  the  atomic  weights  of  elements  of  which  we  can  deter- 
mine the  specific  heat.  If  the  constant  obtained  by  mul- 
tiplying the  specific  heats  by  the  atomic  weights  is  6.25, 
which  is  about  the  average  of  the  different  values  found, 
then  it  is  plain  that,  if  this  number  is  divided  by  the  spe- 
cific heat  of  an  element,  a  number  will  be  obtained  which 
will  approximately  represent  the  atomic  weight.  If  A 
represents  the  atomic  weight,  and  H  the  specific  heat,  the 
following  formula  expresses  the  relation : — 

6.25 
A=-H 

In  order  that  this  might  hold  good  for  all  the  elements 
investigated  by  Dulong  and  Petit,  they  found  it  necessary 
to  change  the  atomic  weights  of  four  of  the  metals;  just 
as  it  had  been  necessary  to  change  certain  of  the  atomic 
weights  in  order  that  Avogadro's  hypothesis  might  hold 
good  in  all  cases ;  but  as  these  atomic  weights  rested  upon 
a  questionable  basis,  there  could  be  no  serious  objection 
to  the  change.  Notwithstanding  the  simplicity  of  the  law, 
its  validity  was  not  immediately  acknowledged. 

Investigations  of  Neumann  and  Regnault. — Twelve  years 
later  (1831)  Neumann  published  investigations  on  the 
specific  heat  of  chemical  compounds,  and  showed  that,  for 
compounds  of  similar  composition,  the  specific  heats  are 
inversely  proportional  to  the  molecular  weights  of  the 
compounds,  or  the  molecules  of  different  compounds  have 
equal  capacity  for  heat — i.  e.,  for  compounds  of  similar 
composition  the  product  of  the  molecular  weight  (M) 
by  the  specific  heat  (H)  is  a  constant  quantity.  For  ex- 
ample, the  specific  heat  of  lead  iodide  is  0.0427 ;  that  of 
lead  bromide  is  0.0533 ;  that  of  lead  chloride  is  0.0664; 
the  molecular  weights  of  these  compounds  are  respectively 
459.48,  365.92,  and  277.14.  The  products  M  X  H  are  as 
follows : — 


68       PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

For  lead  iodide     .        .        .     459.48  X  0.0427  =  19.62 

"        bromide          .         .     365.92  X  0  0533  =  19.50 

chloride         .        .     277.14X00664  =  18.40 

Further,  the  specific  heat  of  barium  chloride  is  0.0902  ; 
that  of  strontium  chloride  is  0.1199 ;  that  of  calcium  chlo- 
ride is  0.1642.  The  molecular  weights  of  these  compounds 
are  respectively  207.64, 158.04,  and  110.65.  The  products 
M  x  If  are : — 

For  barium  chloride  .  .  207.64  X  0.0902  =  18.73 
"  strontium  chloride  .  158.04X0.1199  =  18.95 
"  calcium  chloride  .  .  110.06  X  0  1642  =  18.07 

Subsequently,  similar  investigations  were  carried  out  in 
connection  with  a  large  number  of  compounds,  and  it  is 
particularly  to  the  labors  of  Regnault  (1840)  that  the  de- 
velopment of  this  branch  of  the  subject  is  due.  The  result 
attained  may  be  stated  concisely  thus  :  The  elements  possess 
essentially  the  same  specific  heat,  whether  they  exist  in  a  free 
state  or  in  combination. 

To  show  how  this  conclusion  may  be  drawn  from  known 
facts,  let  us  take  again  the  case  of  lead  iodide.  Lead  has 
the  specific  heat  0.0307,  iodine  0.0541.  Multiplying  by 
the  atomic  weights,  we  have  0.0307  X  206.4  =  6.34 ;  and 
0.0541  X  126.54  =  6.85;  but,  as  can  be  determined,  there 
are  two  atoms  of  iodine  in  the  molecule  of  lead  iodide, 
hence  the  atomic  heat  6,85  must  be  multiplied  by  2,  which 
gives  13.70.  To  raise  the  constituents  of  lead  iodide  one 
degree  in  temperature  would  then  require  an  amount  of 
heat  represented  by  the  number  6.34  -4-  13.70  =  20.04,  and 
we  have  found  that  the  amount  of  heat  necessary  to  raise 
lead  iodide  as  a  compound  one  degree  in  temperature  is 
19.62  As  these  figures  are  practically  identical,  it  follows 
that  the  specific  heat  of  the  elements  in  this  case  is  the 
same,  whether  the  elements  are  in  combination  or  in  the 
free  state. 

Determination  of  Atomic  Weights  by  a  Study  of  the  Spe- 
cific Heat  of  Compounds. — It  thus  appears  that  a  study  of 
the  specific  heat  of  compounds  may  be  of  assistance  in  the 
determination  of  atomic  weights ;  for  the  specific  heat  of 
an  element  can  be  ascertained  even  when  this  cannot  be 
determined  directly.  It  is  difficult,  for  instance,  to  ascer- 
tain the  specific  heat  of  gaseous  elements  directly,  and  yet, 


SOLID  ELEMENTS  AND  COMPOUNDS.  69 

as  these  elements  form  solid  compounds,  the  specific  heat 
of  the  latter  can  be  determined,  and  thus,  indirectly,  that 
of  the  gaseous  elements. 

To  illustrate  by  an  example,  take  the  case  of  chlorine. 
Suppose  it  is  required  to  determine  the  atomic  weight  of  this 
element  by  means  of  determinations  of  specific  heat.  We 
cannot  determine  the  specific  heat  of  the  element  directly. 
It  forms  compounds,  however,  with  other  elements,  the 
specific  heats  and  atomic  weights  of  which  can  be  deter- 
mined. It  combines  with  lead.  The  specific  heat  of  lead 
is  0.0307,  which,  according  to  the  law  of  Dulong  and  Petit, 
gives  the  atomic  weight  206.4.  Now,  in  lead  chloride, 
206.4  parts  by  weight  of  lead  are  combined  with  70.74 
parts  by  weight  of  chlorine  ;  or,  with  1  atom  of  lead  there 
is  combined  an  amount  of  chlorine  weighing  70.74  times 
as  much  as  1  atom  of  hydrogen.  But  we  do  not  know  how 
many  atoms  of  chlorine  this  weight  represents.  It  cannot 
be  less  than  one,  but  it  may  be  2,  3,  4,  or  more  atoms.  We 
determine  the  specific  heat  of  lead  chloride,  and  find  it  to 
be  0.0664.  We  have  assumed  that  the  molecular  heat  of 
a  compound  (i.  e.,  the  product  of  the  molecular  weight  by 
the  specific  heat)  is  equal  to  the  sum  of  the  atomic  heats 
(i.  e.,  the  product  of  the  atomic  weight  by  the  specific  heat) 
of  the  atoms  contained  in  the  compound  ;  or 

MX  H=A  x  H+A'  x  H'  -f  A"  x  H"  ____ 

But,  as  the  products  J.  x  H,  A'  x  H,  A1'  x  H"  have  been 
shown  to  be  constant  and  equal  to  about  6.25,  this  equation 
becomes  — 

MX  H=n6.25-, 

and  from  this  equation,  If  and  IT  being  known,  the  value 
of  n,  or  the  number  of  atoms  contained  in  the  molecule,  can 
be  deduced. 

In  the  case  under  consideration  — 

277.14  x  0.0664  =  n6.25; 
18.40  =  n6.25 


The  conclusion  is,  therefore,  drawn  that  in  the  molecule 
(277.14  parts)  of  lead  chloride  there  are  contained  three 
atoms.  But  we  know  that  there  is  one  atom  of  hea£.; 
hence,  there  must  be  two  atoms  of  chlorine  ;  and,  as  two 


70       PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

atoms  weigh  70.74,  the  atomic  weight  of  chlorine  is  35.37, 
the  same  as  that  found  by  means  of  Avogadro's  method. 

Further,  there  are  other  compounds  in  each  of  which 
70.74  parts  by  weight  of  chlorine  are  combined  with  a  cer- 
tain quantity  of  another  element,  the  molecular  heat  of 
which  is  the  same  as  that  of  lead  chloride.  From  the 
latter  fact  we  conclude  that  there  are  also  three  atoms  con- 
tained in  the  molecules  of  these  compounds,  and,  hence, 
that  quantity  of  an  element  which,  in  these  compounds,  is 
combined  with  70.74  parts  by  weight  of  chlorine,  repre- 
sents the  atomic  weight.  Thus,  the  molecular  heats  of 
barium,  strontium,  and  calcium  chlorides  are  18.73,  18.95, 
and  18.07,  respectively,  numbers  which  are  practically 
identical  with  18.40,  the  molecular  heat  of  lead  chloride ; 
and  in  these  compounds  there  are  136.9  parts  by  weight 
of  barium,  87.3  of  strontium,  and  39.91  of  calcium,  com- 
bined with  70.74  of  chlorine.  The  conclusion  is,  there- 
fore, drawn  that  136.9,  87.3,  and  39.91  are  respectively 
the  atomic  weights  of  barium,  strontium,  and  calcium, 
although  direct  determinations  of  the  specific  heat  have 
been  made  in  only  two  of  these  cases. 

The  following  tables  (I.)  of  elements  and  (II.)  of  com- 
pounds contain  the  numbers  actually  obtained  and  the 
conclusions  drawn  from  them.  The  numbers  under  H 
represent  the  specific  heats  of  the  elements ;  those  under  A 
are  the  atomic  weights  as  determined  by  analytical  methods, 
aided  by  the  rule  of  Avogadro,  or  that  of  Dulong  and 
Petit;  finally,  in  the  last  column  is  the  product  of  the 
atomic  weight  by  the  corresponding  specific  heat  (A  X  H], 
called,  for  convenience,  the  atomic  heat.  The  atomic 
weights  given  in  this  table  are  those  most  commonly  used, 
as  for  the  purposes  in  view  here  they  answer  as  well  as  the 
more  accurate  ones,  and  for  purposes  of  calculation  they 
are  plainly  more  convenient : — 


SOLID  ELEMENTS  AND  COMPOUNDS. 


71 


H. 

A. 

AXH. 

Lithium 
Sodium 
Magnesium    . 
Aluminium   . 
Silicon  . 
Phosphorus  . 
Sulphur 
Potassium 
Calcium 
Chromium     . 
Manganese    . 

0.941 
0.293 
0.250 
0.214 
0.173 
0.174 
0.178 
0.166 
0.170 
0.100 
0.122 
0.114 

7 
23 
24 
27 
28 
31 
32 
39 
40 
52.4 
55 
56 

6.6 
6.7 
6.0 
5.8 
4.8 
5.4 
5.7 
6.5 
6.8 
5.2 
6.7 
6.4 

Cobalt  

0.107 

59 

6.2 

Nickel  

0.109 

59 

6.4 

Copper  ..... 
Zinc      

0.0952 
0.0955 

63.1 
65 

6.0 
6.2 

Arsenic          .... 
Selenium       .... 
Bromine  (solid)     . 
Molybdenum 
Kuthenium   .... 
Khodium       .... 
Palladium     .... 

0.0814 
0.0746 
0.0843 
0.0722 
0.0611 
0.0580 
0.0593 
00570 

75 
79 
80 
96 
103.5 
104.1 
1062 
108 

6.1 
5.9 

6.7 
6.9 
6.3 
6.0 
6.3 
6.2 

Cadmium      .        .        . 

0.0567 
0.0570 

112 
113.4 

6.4 
6.5 

Tin        

0.0562 

117.4 

6.6 

Antimony     .... 

0.0508 
0.0541 

120 
126.5 

6.1 

6.8 

Tellurium     .... 
Tungsten       .... 
Gold      
Platinum 
Iridium         .         .         ,         , 
Osmium 
Mercury 
Thallium       .        .        .       .. 
Lead     .        .        . 
Bismuth      .  .    •  .    . 

00474 
00334 
0.0324 
0.0324 
0.0326 
0.0311 
0.0317 
0.0335 
0.0307 
0.0308 

125 
184 
197 
194.3 
192.5 
195 
200 
204 
207 
207.3 

5.9 
6.1 
6.4 
63 
63 
6.1 
6.3 
6.8 
6.4 
6.5 

The  following  are  some  of  the  compounds  that  have 
been  employed  for  the  purpose  of  determining  the  atomic 
weights  of  elements.  The  numbers  under  H  are  those 
representing  the  specific  heats  of  the  compounds ;  those 
under  M  are  the  molecular  weights ;  the  products  M  X  JET 


72       PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 


are  the  so-called  molecular  heats  ;  n  represents  the  number 
of  atoms  in  the  molecule  of  the  compound : — 


H. 

M. 

M  V  H 

W» 

MXH. 

ML    )\    .a. 

n. 

CoAs2 

0.0920 

209 

192 

3 

6.4 

Ag2S 

0.0746 

248 

18.5 

3 

6.2 

Cu2S 

0.1212 

158.8 

19.2 

3 

6.4 

HgS 

0.052 

232 

12.1 

2 

6.1 

MS 

0.1281 

91 

11.6 

2 

5.8 

PbS 

0.053 

239 

12.7 

2 

64 

SnS 

0.0837 

150 

12.6 

2 

63 

SnS2 
AgCl 

0.1193 
0.0911 

182 
143.5 

21.7 
13.1 

3 
2 

7.2 
6.6 

CuCl 

0.1383 

98.9 

13.7 

2 

6.9 

KC1 

0.1730 

74.6 

12.9 

2 

6.5 

LiCl 

0.2821 

42.5 

12.0 

2 

6 

NaCl 

0.2140 

58.5 

12.5 

2 

6.3 

BaCl2 

0.0896 

208 

186 

3 

6.2 

CaCL 

0.1642 

111 

18.2 

3 

6.1 

SrCl2 

0.1199 

158.5 

19.0 

3 

6.3 

HgCl2 

00689 

271 

18.7 

3 

6.2 

MgCl2 

0.1946 

95 

18.5 

3 

6.2 

MnCL 

0.1425 

126 

18.0 

3 

60 

PbCl2 

0.0664 

278 

18.5 

3 

6.2 

Exceptions  to  the  Law  of  Dulong  and  Petit. — An  ex- 
amination of  these  tables  shows  that  the  product,  A  x  -ET,  in 

the  first,  and  the  quotient,  —      — ,  in  the  second — although 

K\J 

assumed  to  be  constant  in  value — vary  considerably  from 
the  mean  value,  6.25.  Some  of  the  variations  are,  no 
doubt,  due  to  errors  of  observation  in  consequence  of  the 
imperfections  of  the  methods  employed  for  the  determina- 
tion of  specific  heat.  Indeed,  in  all  the  cases  cited  in  the 
above  tables  the  variations  are  hardly  great  enough  to  lead 
us  to  suspect  the  incorrectness  of  the  law  of  Dulong  and 
Petit.  The  elements  carbon,  boron,  and  silicon,  however, 
give  results  that  are  not  in  harmony  with  the  law  as  stated. 
This  will  be  seen  best  by  means  of  the  following  table,  in 
which  His  the  specific  heat;  t,  the  temperature  at  which 
the  determination  was  made;  A,  the  atomic  weight ;  and 
A  x  -ET,  the  atomic  heat.  As  these  three  elements  form 


SOLID  ELEMENTS  AND  COMPOUNDS. 


73 


the  most  marked  exceptions  to  the  law,  all  the  more  reliable 
determinations  of  their  specific  heats  are  given : — 


H. 

t. 

A. 

AXH. 

Carbon 

a. 

Diamond    .     . 

0.064 

—50.5° 

12 

0.77 

a 

0.096 

—10.6° 

12 

1.15 

u 

0.113 

+10.7° 

12 

1.36 

a 

0.132 

33.4° 

12 

1.58 

tt 

0.153 

58.4° 

12 

1.84 

«  i 

0.177 

85.5° 

12 

2.12 

t 

0.222 

140° 

12 

2.66 

1  1 

0.273 

206.1° 

12 

3.28 

<  < 

0.303 

247° 

12 

3.64 

a 

0.441 

606.7° 

12 

5.29 

tt 

0.449 

806.5° 

12 

5.39 

ii 

0.459 

985° 

12 

5.51 

b. 

Graphite    .     . 

0.114 
0.199 

—50.3° 
61.3° 

12 
12 

1.37 
2.39 

ri 

0.297 

201.6° 

12 

3.56 

u 

0445 

641.9° 

12 

5.34 

n 

0.467 

977.9° 

12 

5.60 

c. 

Charcoal     .     . 

0.194 

0°to    99.2° 

12 

2.33 

n 

0.239 

0°  to  223.6° 

12 

2.87 

Silicon 

Crystallized    . 

0.136 

—39.8° 

28 

3.81 

a 

0.170 

+21.6° 

28 

4.76 

<. 

0.183 

57.1° 

28 

5.12 

it 

0190 

86° 

28 

5.32 

(i 

0.196 

128.7° 

28 

5.49 

« 

0.201 

184.3° 

28 

5.63 

a 

0.203 

232.4° 

28 

5.68- 

Boron 

Crystallized*  .     . 

0.192 

—39.6° 

11 

2.11 

(i 

0.238 

+26.6° 

11 

2.62 

ii 

0.274 

76.7° 

11 

3.01 

« 

0307 

125.8° 

11 

3.38 

a 

0338 

177.2° 

11 

3.72 

(C 

0.366 

233.2° 

11         4.03 

An  examination  of  the  above  table  shows  that  at  ordi- 
nary temperatures  the  elements  carbon,  silicon,  and  boron 

*  As  has  been  shown  by  Hampe,  crystallized  boron,  whether  the 
crystals  are  of  the  black  or  yellow  variety,  is  not  the  pure  ele- 
ment. According  to  this  chemist,  the  black  crystals  have  the  com- 
position A1B12,  and  the  yellow  crystals  the  composition  C2A13B48. 


74      PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

do  not  follow  the  law  of  Dulong  and  Petit,  as  it  has  been 
stated.  It  will  be  seen,  however,  that  as  the  temperature 
is  raised  the  specific  heat  becomes  greater,  until  finally,  in 
the  cases  of  carbon  and  silicon,  a  point  is  reached  beyond 
which  there  is  no  marked  change.  Thus,  at  600°  the  spe- 
cific heat  of  diamond  is  0.441,  and  at  985°  it  is  0.449. 
That  of  silicon  is  0.201  at  185°,  and  0.203  at  332°.  At 
these  temperatures  the  elements  obey  the  law  of  Dulong 
and  Petit.  In  the  case  of  boron  it  will  be  observed  that 
the  highest  temperature  at  which  a  determination  of  the 
specific  heat  has  actually  been  made  is  233.2°.  Assuming 
that  the  rate  of  increase  above  230°  is  the  same  as  the 
rate  between  — 40°  and  230°,  the  specific  heat  would  be- 
come 0.50  at  about  600°,  and  this  is  the  figure  required  by 
the  law.  The  above  facts  may  be  stated  in  this  form: 
The  specific  heats  of  the  elements  carbon,  silicon,  and 
boron  increase  gradually  from  the  lowest  temperature  to 
certain  points,  above  which  they  remain  practically  con- 
stant. The  point  for  carbon  and  boron  is  about  600° ;  that 
for  silicon  about  200°.  At  these  temperatures,  and  above 
them,  the  elements  have  the  following  specific  heats :  car- 
bon 0.46;  boron  0.50;  silicon  0.205.  The  products  ob- 
tained by  multiplying  these  figures  by  the  atomic  weights 
12,  11,  and  28  are  5.5,  5.5,  and  5.8;  so  that  carbon,  boron, 
and  silicon  are  not  exceptions.  The  law  of  Dulong  and 
Petit,  however,  is  a  little  more  complicated  than  as  stated 
above,  and  should  have  the  following  form : 

The  specific  heats  of  the  elements  vary  with  the  temper- 
ature; but  for  every  element  there  is  a  point,  T,  above 
which  the  variations  are  very  slight.  The  product  of  the 
atomic  weight  by  the  constant  value  of  the  specific  heat  is 
nearly  a  constant,  lying  between  5.5  and  6.5. 

Results  similar  to  those  above  discussed  have  been  ob- 
tained by  Nilson,  Pettersson,  and  Humpidge  in  the  case  of 
beryllium. 

To  account  for  the  variations  in  the  specific  heats  of  the 
elements,  it  has  been  suggested  that  in  most  cases  we  can- 
not determine  the  true  specific  heat.  This  is  only  that 
heat  which  goes  to  increase  the  temperature.  In  measur- 
ing specific  heats  we  have  to  deal  with  a  complex  quantity, 
viz.,  that  heat  which  raises  the  temperature,  together  with 
that  which  performs  internal  work  and  that  which  performs 
external  work.  In  the  case  of  solids  and  liquids  the  ex- 


SOLID  ELEMENTS  AND  COMPOUNDS.  75 

ternal  work  performed  is  very  small.  The  internal  work 
is  probably  different  in  different  cases,  and  may  amount  to 
considerable.  The  fact  that  the  specific  heats  of  so  many 
elements,  when  multiplied  by  the  atomic  weights  of  these 
elements,  give  the  same  product,  indicates  that  in  these 
cases  the  internal  work,  like  the  specific  heat,  is  inversely 
proportional  to  the  atomic  weights.  It  is  evident,  accord- 
ing to  this,  that,  if  the  amount  of  internal  work  varies  in 
different  elements,  the  specific  heat  will  also  vary  in  such 
a  way  as  to  seem  to  conflict  with  the  law.  It  remains, 
then  to  be  shown  how  determinations  of  specific  heat  can 
be  made  which  shall  be  independent  of  the  internal  and 
external  work.  When  this  can  be  done,  it  is  probable 
that  the  law  of  Dulong  and  Petit  will  be  found  to  be  a 
perfect  law  without  exceptions. 

ISOMORPHISM  AS  FURNISHING  A  MEANS  FOR  DETER- 
MINING ATOMIC  WEIGHTS. — Another  means,  once  con- 
sidered valuable,  for  determining  atomic  weights  is  found 
in  the  phenomena  of  isomorphism.  It  has  long  been  known 
that  some  substances  of  entirely  different  composition  have 
approximately  the  same  crystalline  form.  This  was  ex- 
plained by  Mitscherlich  (1819)  by  supposing  that  an  equal 
number  of  atoms  in  different  molecules  causes  the  same 
crystalline  form.  A  little  later  he  proposed  the  following 
law  of  isomorphism : 

An  equal  number  of  atoms,  united  in  the  same  way, 
give  the  same  crystalline  form ;  and  this  crystalline  form 
is  independent  of  the  chemical  nature  of  the  atoms,  being 
only  dependent  on  their  number  and  arrangement. 

If  this  law  were  strictly  true,  it  is  plain  that  we  should 
in  many  cases  be  able  to  determine  atomic  weights  by  its 
aid.  A  few  examples  will  illustrate  the  method. 

The  two  substances  BaCl2  +  2H2O  and  BaBr2  -f  2H2O 
are  isomorphous.  We  may  assume,  then,  that  their  mole- 
cules contain  the  same  number  of  atoms,  and,  if  we  know 
the  atomic  weights  of  the  constituents  of  the  molecule 
BaCl2  -f  2H2O,  we  can  easily  determine  the  atomic  weight 
of  the  constituent  bromine  in  the  molecule  BaBr2  -f-  2H2O. 
Further,  the  compounds  CuAgS  and  CuCuS  are  isomor- 
phous. If  the  molecules  of  each  contain  the  same  number 
of  atoms,  and  the  atomic  weights  of  copper  and  sulphur  are 
known,  the  atomic  weight  of  silver  can  be  found. 


76       PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

This  method  has  no  doubt  been  of  service  in  the  deter- 
mination of  atomic  weights.  Nevertheless,  it  requires  but 
a  few  examples  to  prove  that,  if  to  the  term  isomorphism 
is  given  the  wide  meaning  implied  in  the  wording  of  the 
law  as  stated  above,  the  results  reached  by  means  of  it  are 
not  reliable.  The  salts  BaMn2O8,  Na2SO4,  and  Na2SeO4 
have  the  same  crystalline  form,  and  yet  the  best  methods 
for  determining  formulas  show  that  those  given  are  prob- 
ably correct.  If  we  were,  in  these  cases,  to  assume  that 
the  number  of  atoms  in  each  of  the  molecules  is  the  same, 
we  should  reach  results  at  variance  with  those  obtained  by 
the  most  reliable  methods.  It  will  thus  be  seen  that  if  the 
isomorphism  of  salts  is  employed  as  a  method  for  the  deter- 
mination of  atomic  weights,  the  results  must  be  looked  upon 
as  doubtful,  unless  the  sense  of  the  word  isomorphism  is 
.restricted. 

According  to  Kopp,  identity  of  crystalline  form  is  not  a 
sufficient  basis  for  designating  compounds  as  isomorphous, 
and  the  term  should  be  used  in  a  much  more  restricted 
sense.  If,  according  to  this  writer,  a  crystallized  com- 
pound has  the  power  of  growing  in  a  solution  of  another 
compound,  then  the  two  may  be  regarded  as  isomorphous. 
Thus,  as  is  well  known,  if  a  crystal  of  ordinary  alum  is 
placed  in  a  solution  of  iron  alum,  it  will  grow  in  the  same 
way  that  it  would  in  the  original  solution.  This  power  of 
forming  overgrowths  should  be  regarded  as  the  true  criterion 
of  isomorphism.  Understood  in  this  sense,  the  use  of  the 
expression  isomorphism  becomes  much  more  restricted  than 
it  was  when  used  in  the  old  sense.  There  are  many  com- 
pounds that  have  the  same  crystalline  form  and  are  not 
isomorphous  in  the  new  sense. 


In  the  foregoing,  free  use  has  been  made  of  the  expres- 
sion Atomic  weights.  Using  the  language  of  the  atomic 
theory,  the  methods  for  determining  the  Atomic  weights 
have  been  considered.  It  is  well,  however,  before  passing 
on  to  the  next  part  of  our  subject  to  recall  the  fact  that  it 
cannot  be  positively  asserted  of  the  figures  called  atomic 
weights  that  they  represent  the  relative  weights  of  atoms. 
Whether  they  do  or  do  not,  they  certainly  have  a  close 
relation  to  certain  important  facts  that  lie  at  the  foundation 
of  all  cases  of  chemical  action.  The  laws  of  definite  and 


SOLID  ELEMENTS  AND  COMPOUNDS.  77 

multiple  proportions  are  the  fundamental  laws  of  chemistry. 
They  are  as  firmly  established  as  any  laws  of  nature,  and 
they  are  simply  the  expression  of  facts  observed.  We  can 
certainly  select  for  each  element  a  figure,  which  itself  will 
represent,  or  a  multiple  of  which  will  represent,  the  propor- 
tion by  weight  in  which  this  element  enters  into  chemical 
action.  The  difficulty  is  to  know  which  one  to  select.  To 
get  over  the  difficulty  we  study  certain  physical  properties 
of  the  elements  and  their  compounds,  as  the  specific  gravity 
of  the  vapor,  the  specific  heat,  the  crystalline  form,  and 
determine  the  weights  of  the  substances  which,  on  this 
basis,  seem  to  be  analogous.  We  thus  get  figures  which 
bear  to  one  another  relations  very  similar  to  those  which  the 
so-called  atomic  weights,  determined  by  chemical  methods, 
bear  to  one  another.  The  similarity  of  the  relations  of  the 
figures  obtained  by  the  different  methods  suggests  a  common 
cause,  and  we  attempt  to  satisfy  our  desire  to  ascertain  the 
cause  by  saying  that  the  figures  represent  the  relative 
weights  of  atoms;  but  the  figures  would  be  just  as  valu- 
able without  this  theory,  and  we  might  deal  with  all  the 
facts  of  chemistry  without  referring  to  the  possible  exist- 
ence of  atoms.  We  shall  next  find  that  the  atomic  weights 
determined  by  the  methods  of  Avogadro  and  Dulong  and 
Petit  are  confirmed  in  a  most  striking  way  by  an  extremely 
important  discovery,  touching  the  relations  between  the 
atomic  weights  of  the  elements  and  their  properties. 


78      PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 


CHAPTER    VI. 

PROPERTIES  OF  THE  ELEMENTS  AS   FUNCTIONS    OF   THEIR 
ATOMIC   WEIGHTS — THE   PERIODIC    LAW. 

Natural  Groups  of  Elements. — On  examining  the  list  of 
elements  and  their  atomic  weights,  we  find  that  there  are  a 
number  of  well-marked  groups,  indicating  some  connection 
between  the  atomic  weights  and  properties  of  the  elements. 
Among  these  may  be  mentioned  chlorine,  bromine,  and 
ipdine;  sulphur,  selenium,  and  tellurium;  lithium,  sodium, 
and  potassium.  Arranging  these  according  to  their  atomic 
weights  we  have : — 

Cl        35.37  8        31.98  Li        7.01 

Br        79.76  Se       78.87  Na     23 

I        126.54  Te     125  K      39.03 

If  in  each  of  these  groups  the  atomic  weights  of  the  first 
and  last  members  are  added  together,  and  the  sum  divided 
by  2,  very  nearly  the  atomic  weights  of  the  middle  mem- 
bers of  the  series  are  obtained: ^ —  —==80.95, 

31.98  +  125      _        7.01  +  39.03 

—  =  78.49,  -  -  =  23.02.     It  will  be 

L  & 

observed,  also,  that  the  elements,  whose  atomic  weights  are 
thus  closely  connected,  are  themselves  very  closely  allied 
in  their  properties.  Considerations  of  this  kind  have  led 
chemists,  from  time  to  time,  to  examine  the  atomic  weights 
more  closely,  and  as  a  result  of  these  examinations  it  has 
been  found  that  the  connection  above  indicated  is  much 
more  general  than  was  at  first  supposed.  Attention  was 
first  called  to  the  general  character  of  the  relations  between 
the  properties  of  the  elements  and  their  atomic  weights  by 
J.  A.  R.  Newlands,  in  a  number  of  papers  which  appeared 
in  the  years  1864-66,  but,  owing  to  some  marked  inconsis- 
tencies in  his  arrangement  of  the  elements,  the  importance 
of  the  subject  was  not  generally  recognized.  Shortly  after- 
ward, in  1869  and  1870,  two  articles  appeared,  one  by  I). 


THE  PERIODIC  LA  W.  79 

Mendeleeff*  and  the  other  by  Lothar  Meyer,f  in  which 
these  relations  were  treated  in  a  masterly  manner,  and  it 
was  then  seen  that  one  of  the  most  important  laws  of 
chemistry  had  been  discovered.  Everything  learned  since 
then  has  only  made  it  appear  more  and  more  certain  that 
the  periodic  law  is  a  fundamental  law  of  chemistry. 

Mendeleeff's  Tables.  —  Mendeleeff  first  calls  attention  to 
the  fact  that,  if  all  light  elements  with  atomic  weights  from 
7  to  36J  are  arranged  in  the  order  of  their  atomic  weights, 
the  following  remarkable  table  is  obtained  :  — 

Li  =   7  ;  Be  =   9  ;  B   =  11  ;  C  =  12  ;  N  =  14  ;  O  =  16  ;  P  =  19  ; 
Na  =  23;  Mg  =  24  ;   Al  =  27  ;  Si  =  28;   P  =31;  S  =32;   Cl  =  35.5 

In  these  two  series  elements  which  we  recognize  as 
similar  come  to  stand  together,  as  lithium  and  sodium, 
beryllium  and  magnesium,  carbon  and  silicon,  oxygen  and 
sulphur,  etc.  The  gradual  change  in  the  properties  of 
the  members  of  the  series,  as  we  pass  from  left  to  right,  is 
noticed  especially  with  respect  to  the  compounds  which 
the  elements  form.  Thus,  only  the  last  four  members  com- 
bine with  hydrogen,  yielding  compounds  of  the  general 
formulas  — 

RH. 


The  character  of  these  hydrogen  compounds  also  changes 
gradually,  according  to  the  position  in  the  series.  Hydro- 
chloric acid,  HC1,  is  a  marked  acid  of  great  stability; 
hydrogen  sulphide,  H2S,  is  a  weak  acid  decomposable  by 
heat  ;  phosphine,  PH3,  is  not  an  acid,  and  is  less  stable  than 
the  preceding  compounds,  and  silicon  hydride,  SiH^,  is  still 
less  stable  than  phosphine,  PH3. 

The  oxides  of  the  members  of  the  second  series  form  this 
series  — 

Na20,    Mg202,     A1A,    SiA,     P2O6,    S2O6,    CIA, 
or  MgO,  or  SiO2,  or  SO3. 

From  left  to  right  in  this  series  the  basic  properties  grow 
weaker  and  the  acid  properties  stronger.  Again,  in  the 

*  Zeitschrift  fiir  Chemie,  1869,  405;  and  Annalen  der  Chemie,  8 
Suppl  ,  133 

f  Annalen  der  Chemie,  7  Suppl.,  356. 

J  To  avoid  unnecessary  complications,  the  ordinary  atomic  weights 
are  used  in  the  treatment  of  this  subject. 


80      PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

composition  of  the  hydroxides  a  similar  regularity  is  ob- 
served — 

Na(OH),  Mg(OH)2,  Al(OH),,  Si(OH)4,  PO(OH)3, 
S02(OH)2,  C10.(OH). 

Another  point  to  be  noted  is  this  :  that,  in  the  series  with 
which  we  are  dealing,  the  metals  are  at  one  end  and  the 
so-called  non-metals  at  the  other,  while  those  elements 
which  are  sometimes  classed  with  the  metals  and  some- 
times with  the  non-metals,  as,  for  instance,  silicon,  come  in 
the  middle. 

But  just  as  the  chemical  properties  undergo  gradual 
change  in  the  series  mentioned,  so,  also,  a  corresponding 
change  is  noticed  in  the  physical  properties.  To  illustrate 
this,  the  specific  gravities  and  the  atomic  volumes  of  the 
members  of  the  second  series  are  given  :  — 


Na 

Mg 

Al 

Si 

P 

s 

Cl 

Spec. 

gr. 

0.97 

1.75 

2.67 

2.49 

1.84 

2.06 

1.33 

Atom, 

,  vol. 

24 

14 

10 

11 

16 

16 

27 

Na20 

Mg202 

A1203 

Si204 

P205 

S«06 

C12O7 

Spec. 

gr- 

2.8 

3.7 

4.0 

2.6 

2.7 

1.9 

? 

Atom 

.  vol. 

22 

22 

25 

45 

55 

82 

? 

Another  series  corresponding  to  the  two  already  given  is 
the  following  :  — 

Ag=108    Cd  =  112    In  =  113    Sn  =  118    Sb  =  120    Te=125    1  =  127 
Sp.gr.     10.5        8.6          7.4          7.2          6.7  6.2          4.9 

All  the  elements  may  be  arranged  in  series  similar  to 
the  above,  and  thus  a  very  intimate  connection  between 
the  atomic  weights  and  the  properties  of  the  elements  is 
shown  to  exist.  It  will  be  noticed  that  the  changes  in  the 
properties  of  the  elements  are  periodic.  First,  these  prop- 
erties change  according  to  the  increasing  atomic  weights, 
then  they  are  repeated  in  a  new  period  with  the  same  regu- 
larity as  in  the  preceding  series.  Such  series  as  those 
already  mentioned  are  called  small  periods.  If  hydrogen 
is  placed  in  the  first  series,  then  lithium,  etc.,  come  in  the 
second  series,  sodium,  etc.,  in  the  third,  etc. 

But  all  the  known  elements  cannot  be  arranged  in  the 
small  periods,  and,  what  is  much  more  important,  the  cor- 
responding members  of  the  even  (4,  6,  etc.)  periods,  or  of 
the  uneven  (5,  7,  etc.),  resemble  one  another  more  closely 


THE  PERIODIC  LA  W.  81 

than  the  members  of  the  even  periods  resemble  those  of 
the  uneven  periods.  This  is  seen  from  the  following : — 

Fourth  period :  K,    Ca,  Sc,  Ti,  V,  Cr,  Mn. 

Fifth        "       :  Cu,   Zn,  Ga,  Ge,  As,  Se,  Br. 

Sixth        "       :  Rb,  Sr,  Y,  Zr,  Nb,  Mo,  — 

Seventh    "       :  Ag,  Cd,  In,  Sn,  Sb,  Te,  I. 

The  members  of  the  fourth  and  sixth  periods  resemble  one 
another  more  closely  than  they  resemble  the  members  of 
the  fifth  or  seventh  periods;  and  the  members  of  the  fifth 
and  seventh  periods  resemble  one  another  closely.  The 
last  members  of  the  even  periods  resemble  the  first  mem- 
bers of  the  succeeding  uneven  series  in  many  respects.  Thus 
chromium  and  manganese  in  their  basic  oxides  are  similar 
to  copper  and  zinc.  On  the  other  hand,  between  the  last 
members  of  the  uneven  periods  and  the  first  members  of 
the  succeeding  even  periods  there  are  very  marked  differ- 
ences, as,  for  instance,  between  bromine  and  rubidium. 
Further,  between  the  last  members  of  the  even  periods 
and  the  first  members  of  the  uneven  periods  all  those 
elements  which  cannot  be  arranged  in  the  small  periods 
would  naturally  fall  according  to  their  atomic  weights  and 
properties.  Thus,  between  chromium  and  manganese,  on 
the  one  hand,  and  copper  and  zinc  on  the  other,  iron, 
cobalt,  and  nickel  fall;  the  following  series  being  thus 
formed : 

Cr  — 52;  Mn  — 55;  Fe  — 56;  Co  — 58;  Ni  =  59; 
Cu  =  63;  Zn  =  65. 

As  iron,  cobalt,  nickel  follow  the  fourth  period,  so  ruthe- 
nium, rhodium,  palladium  follow  the  sixth  period,  osmium, 
iridium,  platinum  follow  the  tenth  period.  Two  small 
periods  (an  even  and  uneven),  together  with  an  interme- 
diate series  of  the  elements  just  mentioned,  form  a  large 
period.  As  the  intermediate  members  mentioned  corre- 
spond to  none  of  the  seven  small  periods,  they  form  an 
independent  eighth  group : — 

Fe  =  56 ;  Co  —  58 ;  Ni  «  59, 
Ru  =  101;  Rh  — 103;  Pd  —  106, 
Oa=191;  Ir  —192.5;  Pt  — 194.3. 

The  members  of  this  group  resemble  one  another  in  the 
same  way  as  the  corresponding  members  of  the  even 


82      PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

periods,  as,  for  instance,  vanadium,  columbium,  tantalum, 
or  chromium,  molybdenum,  tungsten,  and  others. 

The  two  following  tables  of  Mendeleeff  clearly  show  the 
relations  described.  In  the  first,  the  elements  with  their 
atomic  weights  are  arranged  in  large  periods ;  in  the  second, 
they  are  arranged  in  groups  and  series  in  such  a  manner 
as  distinctly  to  indicate  the  differences  between  the  even 
and  uneven  periods. 


THE  PERIODIC  LAW. 


83 


rH  CO  CO 

o  o  o 


I  I 


1       I 


8 


§     I 

1 

H 

I  I 


s 


00 


S 


w 


' 


o       B 


o 

0  05 
5 


51* 


W 


a 


O5  CO 

rH  r^ 


ff 


PM 


»O  CO 


t^  00          05  O 


84       PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 


I. 

II. 

III. 

IV. 

V. 

VI. 

RaO 

I. 

Li       7 

K     39    Rb   85 

Cs  133 

—     — 

—     — 

RO 

II. 

Be      9 

Ca    40 

Sr    87 

Bal37 

—     — 

—     — 

RaOs 

III. 

B      11 

Sc    44 

Y      89 

La  139 

Ybl73 

_     _ 

RO2 

IV. 

(H4C) 

C       12 

Ti    48 

Zr    90 

Ce  142 

-     - 

Th231 

R205 

V. 

(H,N) 

N      14 

V     51 

Cb    94 

Di  145 

Ta  182 

—     — 

R03 

VI. 

(H20) 

0       16 

Cr    52 

Mo  96 

—     — 

W  184 

U    240 

RA 

VII. 

(HF) 

F      19 

Mn  55 

-     - 

—     — 

-     - 

-     - 

f 

Fe    56 

RulOl 

_     _ 

Os  191 



R04 

VIII. 

J 

Co    59 

Rhl03 

—     — 

Ir    193 

—     — 

I 

Ni    59 

Pdl06 

—     — 

Pt  195 

—     — 

RsO 

I. 

H=  1 

Na=23 

Cu     63 

AglOS 

—     — 

Au  196 

—     — 

RO 

II. 



Mg  24 

Zn    65 

Cdll2 

—     — 

Hg200 

—     — 

R203 

III. 

Al    27 

Ga    70 

In  113 

-     - 

Tl  204 

—     — 

R02 

IV. 

(H4R) 

Si     28 

Ge   72 

Sn  118 

—     — 

Pb207 

_     _ 

Rs06 

V. 

(H3R) 

P      31 

As    75 

Sb  120 

—     — 

Bi  208 

—     — 

RO3 

VI. 

(H2R) 

S      32 

Se    79 

Te  125 

—     — 

—     — 

—     — 

R20T 

VII. 

(HR) 

Cl  35.5 

Br    80 

I     127 

—     — 

—     — 

—     — 

It  is  not  necessary  to  point  out  here  all  the  properties  of 
the  elements  which  have  been  shown  to  vary  in  harmony 
with  the  changes  of  the  atomic  weights.  What  has  already 
been  said  will  suffice  to  indicate  the  principle  involved  in 
the  construction  of  the  tables  of  Mendeleeff.  Close  study 
of  these  tables  does  undoubtedly  show  that  they  contain 
some  imperfections  and  apparent  contradictions  ;  still,  these 
are  not  numerous  enough  nor  serious  enough  to  interfere 
materially  with  the  value  of  the  tables.  It  is  evident  that 
the  first  condition  for  the  construction  of  such  tables  is  the 
correct  determination  of  all  the  atomic  weights.  We  have 
seen  with  what  difficulty  this  determination  is  often  at- 
tended, and  how  doubtful  some  of  the  results  obtained  are. 
When  all  the  atomic  weights  shall  have  been  accurately 
determined,  and  all  the  properties,  both  physical  and  chemi- 
cal, of  the  elements  are  known,  then  a  table  constructed  on 
the  principle  of  the  above  will,  in  all  probability,  show  a 
perfect  accordance  between  atomic  weights  and  properties. 


THE  PERIODIC  LA  W.  85 

Mendel6eff  originally  proposed  to  use  the  periodic  law, 
as  he  calls  the  law  governing  the  variations  in  the  atomic 
weights  and  properties  of  the  elements,  for  the  purpose  of 
determining  the  properties  of  undiscovered  elements.  When 
Table  II.  was  first  constructed  a  member  was  wanting 
between  calcium  and  titanium  in  the  second  series.  The 
atomic  weight  of  this  element  should  be  about  44,  and  its 
properties  were  also  very  nearly  foretold  from  its  position. 
Its  oxide  should  have  the  composition  R2O3,  and  its  proper- 
ties should  bear  the  same  relation  to  aluminium  oxide, 
A12O3,  that  those  of  calcium  oxide,  CaO,  bear  to  magne- 
sium oxide,  MgO,  or  titanium  oxide,  TiO2,  to  silicon  dioxide, 
SiO2.  Consequently  it  should  be  a  more  energetic  base 
than  aluminium  oxide,  A12O3,  and  should  resemble  it  in 
corresponding  compounds.  Its  sulphate  should  not  be  so 
easily  soluble  as  aluminium  sulphate,  because  calcium  sul- 
phate is  more  difficultly  soluble  than  magnesium  sulphate. 
Thus,  throughout  the  whole  list  of  properties  such  com- 
parisons were  made,  and  the  unknown  element  was  more 
accurately  described  than  some  of  those  which  have  been 
known  for  a  long  time. 

The  prediction  of  this  element,  to  which  Mendel4eff  gave 
the  name  ekaboron,  proved  to  be  one  of  the  most  remark- 
able predictions  ever  made  in  the  field  of  chemistry.  Within 
a  few  years  the  metal  scandium  was  discovered,  and  was 
shown  to  be  the  predicted  ekaboron.  Before  its  discovery, 
in  a  similar  way,  gallium  was  predicted  and  described 
under  the  name  of  ekalu minium;  and  later  germanium 
was  discovered  and  shown  to  be  the  element  needed  to  fill 
the  gap  between  gallium  and  arsenic,  the  properties  of 
which,  under  the  name  ekasilicon,  had  been  described  by 
Mendeleeff  nearly  twenty  years  previously. 

Such  speculations  are,  without  doubt,  very  attractive; 
but  we  must  not  forget  that  the  tables  which  are  used  as 
their  foundation  are  more  or  less  imperfect,  and  hence  the 
conclusions  drawn  must  necessarily  be  doubtful.  On  the 
other  hand,  the  time  will  come  when  such  speculations  can 
be  indulged  in  without  risk  of  reaching  doubtful  results. 
The  approach  of  this  time  will  be  hastened  by  just  such 
efforts  as  those  of  Mendeleeff  to  discover  the  law  govern- 
ing the  connection  between  the  atomic  weights  and  the 
properties  of  the  elements. 

5 


86      PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

Lothar  Meyer's  Arrangement  of  the  Elements. — Another 
arrangement  of  the  elements  showing  the  connection  be- 
tween the  atomic  weights  and  the  properties  of  the  elements 
is  that  of  Lothar  Meyer,  already  alluded  to.  His  table* 
follows : — 


I. 

II. 

III. 

IV. 

V. 

VI. 

VII. 

VIII. 

LI 

7.01 

Be 

9.08 

B 

10.9 

C 

Na 

11.97 

N 

23 

Mg 

14.01 

O 

24.3 

Al 

15.96 

P 

27.04 

Si 

K 

28.3 

P 

39  03 

Ca 

30.96 

S 

39.91 

Sc 

31.98 

Cl 

43.97 

Ti 

48 

V 



Cu 

63  18 

Zn 

51.1 

Cr 

65.10 

Ga 

52.45 

Mn 

69.9 

Ge 

Kb 

72.3 

As 

oc  o 

Sr 

74.9 

Se 

87.3 

?  Y 

78.87 

Br 

88.9 

Zr 

•Ag 

90.4 

Cb 

107  66 

Cd 

93.7 

Mo 

111.7 

In 

95.9 

? 

1  —  •  

113.6 

Sn 

98 

Ru       Rh       Pd 

Cs 

118.8 

Sb 
119.6 

Te 



132.7 

136.  9 

La 

125 

I 

? 

138 

Ce 
139.9 

Di? 

165 

? 
170 

Yb 

145 

151 

Au 

172.6 

? 
176 

Ta 

196.7 

Hg 

199.  8 

Tl 

182 

W 

183.6 

? 

-1  OK 

OQ          Tr           Pt 

203.7 

Pb 

? 

206.4 

Bi 

207  3 

? 

222 

? 
226 

? 

210 

? 

230 

?Th 

211 

232 

? 

234 

?U 
239 

This  table  contains  all  the  elements  whose  atomic  weights 
have  been  determined  with  any  degree  of  certainty.     The 


*  Die  modernen  Theorien  der  Chemie.   Fiinfte  Auflage.   Breslau, 

1884. 


THE  PERIODIC  LA  W.  87 

essential  difference  between  this  arrangement  and  the  first, 
one  of  those  of  Mendeleeff  given  above  consists  in  this, 
that  the  periods  are  arranged,  not  in  horizontal  lines,  but 
in  lines  which  are  so  inclined  that,  if  the  table  were  pasted 
on  a  cylinder  of  proper  size,  the  table  would  have  the  form 
of  a  continuous  spiral,  beginning  at  the  top  with  lithium 
and  ending  at  the  bottom  with  uranium.  Meyer  further 
points  out  the  connection  between  the  atomic  weights  and 
the  following  properties  of  the  elements :  specific  gravity 
in  the  solid  condition,  as  shown  by  a  comparison  of  the 
atomic  volumes;  metallic  ductility,  fusibility,  and  vola- 
tility ;  crystalline  form ;  influence  upon  the  refraction  of 
light;  specific  heat;  conducting  power  for  heat  and  elec- 
tricity. The  connection  is  shown  to  be  a  remarkably  close 
one.  For  a  further  study  of  this  subject  the  student  is 
referred  to  Lothar  Meyer's  masterly  book,  Die  modernen 
Theorien  der  Chemie. 

According  to  what  is  thus  far  known  concerning  the 
substances  argon  and  helium,  it  is  impossible  to  arrange 
them  in  the  tables  based  upon  the  periodic  law  in  the  same 
way  that  all  the  other  known  elements  are  arranged.  It 
should  be  borne  in  mind,  however,  that  in  consequence  of 
their  peculiar  properties  our  knowledge  of  these  substances 
is  as  yet  very  imperfect. 


88       PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 


CHAPTER   VII. 

VALENCY. 

Definition. — The  methods  for  the  determination  of  the 
molecular  formulas  of  compounds  have  been  discussed. 
By  applying  these  methods  expressions  are  obtained  for 
the  composition  of  compounds  which,  in  all  probability, 
are  relatively  correct.  If  the  formulas  as  determined  are 
now  compared,  certain  regularities  will  appear.  A  few 
examples  among  the  hydrogen  compounds  will  serve  the 
purpose  r — 

I.  II.  III.  IV. 

C1H,  OH2,  NH3,  CH4, 

BrH,  SH2,  AsH3,          Sifl4. 

IH,  SeH2,  SbH3, 

FH,  TeH2,  PH3, 

It  will  be  seen  that  chlorine,  bromine,  iodine,  and  fluo- 
rine combine  with  hydrogen  in  the  proportion  of  atom  to 
atom;  oxygen,  sulphur,  selenium,  and  tellurium  combine 
with  hydrogen  in  the  proportion  of  two  atoms  of  hydrogen 
to  one  atom  of  the  other  element ;  in  the  compounds  with 
nitrogen,  arsenic,  antimony,  and  phosphorus,  three  atoms 
of  hydrogen  are  in  combination  with  one  atom  of  the  other 
element ;  and,  lastly,  four  atoms  of  hydrogen  are  in  com- 
bination with  one  atom  of  carbon  and  of  silicon.  No  ele- 
ments are  known  which  combine  with  a  larger  proportion 
of  hydrogen  than  carbon  and  silicon.  It  is  clear,  there- 
fore, that  those  elements  which  combine  with  hydrogen  can 
be  divided  into  four  classes,  the  differences  between  which 
are  shown  by  the  examples  given  above.  With  reference 
to  their  power  of  combining  with  the  halogens,  elements 
differ  from  one  another  in  a  similar  way.  The  following 
examples  will  make  this  clear: — 

NaCl    Bad,     PCI,      CCJ4      PC15      WC16 
KC1      CaCl2     AsCl3    SiCl4     AsCl5     — 
LiCl     ZnCJ,    SbCl3    SnCl,    SbC!5      — . 


VALENCY.  89 

There  is  greater  diversity  among  the  compounds  with 
the  halogens  than  among  the  compounds  with  hydrogen. 
While  the  largest  number  of  hydrogen  atoms  with  which  any 
other  atom  can  combine  is  apparently  four,  the  largest  num- 
ber of  chlorine  atoms  with  which  any  other  atom  can  combine 
is  apparently  six.  The  same  is  true  of  the  other  halogens. 
Turning  now  to  the  compounds  of  oxygen  still  greater 
diversity  appears,  as  is  seen  in  the  following  table :  — 

Na,O  MgO  A12O3  SiO2  P2O5  SO3  C12O7  RuO4 
(Mg202)  (Si204)  (SA)  (Ru208) 

K2O  CaO  Se2O8  TiO2  V205  CrO3  Mn2O7  OsO4 

(Ca202)  (TiA)  (Cr206)  (Os2O8) 

The  simplest  compounds  of  oxygen  are  those  in  which 
one  atom  of  this  element  is  in  combination  with  two  atoms 
of  some  other  elements.  From  the  examples  given  above 
it  will  be  seen  that  there  are  eight  classes  of  oxygen  com- 
pounds in  which  the  oxygen  combined  with  two  atoms  of 
the  other  elements  varies  from  one  to  eight  atoms. 

In  general,  then,  it  may  be  said  that  the  elements  differ 
from  one  another  as  regards  the  complexity  of  the  com- 
pounds which  they  can  form  with  other  elements. 

But  there  must  be  some  reason  for  the  difference.  Where 
shall  we  seek  for  it  ?  If  we  accept  the  atomic  hypothesis 
of  Dalton,  we  must  seek  for  the  proximate  causes  of  phe- 
nomena" presented  to  us  by  masses  of  elements  or  their 
compounds  in  the  atoms  composing  these  masses.  Here, 
then,  we  are  to  find  in  the  atoms  themselves  the  proximate 
cause  of  this  new  property  of  elements.  As,  however,  we 
can  learn  nothing  of  atoms  directly,  but  only  of  masses  of 
atoms,  it  is  evident  that  this  cause  cannot  be  discovered, 
but  must  finally  be  imagined,  just  as  the  atom  itself  is  still 
only  imagined  to  exist.  In  other  words,  our  conception 
of  atoms  must  become  enlarged  in  such  a  way  as  to 
account  for  the  new  property,  or  a  subordinate  hypothesis 
must  be  framed  to  supplement  the  atomic  hypothesis. 
Before  framing  this  hypothesis,  let  us  see  whether  we  can 
learn  anything  more  in  regard  to  the  new  property  than 
we  have  yet  learned. 

We  have  seen  above  that  the  molecules  of  some  of  the 
hydrogen  compounds  contain  only  1  atom  of  hydrogen  to 
each  atom  of  the  other  element ;  in  the  molecules  of  other 
compounds  we  find  2  atoms  of  hydrogen  to  1  atom  of  the 


90       PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

other  constituent ;  in  others  3,  and  in  still  others  4  atoms 
of  hydrogen  combined  with  1  atom  of  some  other  element. 
In  thus  stating  the  number  of  atoms  present  in  the  mole- 
cules we  have,  of  course,  left  the  domain  of  facts  and  have 
already  entered  that  of  hypothesis;  still,  the  hypothesis 
involved  in  a  molecular  chemical  formula,  which  has  been 
determined  by  the  aid  of  all  means  at  our  command,  is  one 
which  we  are  justified  in  employing,  and  we  consequently 
state  the  knowledge  we  have  gained  in  terms  of  the  hy- 
pothesis. Taking  into  consideration  the  sum  total  of  our 
knowledge,  as  thus  far  stated,  the  simplest  hypothesis 
which  it  is  possible  to  form  concerning  the  cause  which 
we  are  striving  to  find  is  the  following:  Every  atom  of 
an  element  has  an  inherent  power  of  holding  in  combina- 
tion a  certain  number  of  other  atoms,  this  number  being 
dependent  upon  the  nature  of  the  atoms  held  in  combi- 
nation. The  simplest  atoms  represent  the  unit  of  this 
power,  and  we  can  distinguish  between  these  simplest  or 
unit-atoms  and  such  as  have  the  power  of  holding  in  com- 
bination 2,  3,  4,  or  more  unit-atoms. 

Name  of  the  New  Property. — The  property  of  the  ele- 
ments under  consideration  has,  in  accordance  with  the 
simple  hypothesis  just  given,  been  termed  atomicity,  quan- 
tivalence,  or  only  valency*  The  elements,  which  consist  of 
the  unit-atoms,  are  hence  called  monatomic  or  univalent ; 
those  consisting  of  atoms  which  have  twice  the  combining 
power  of  the  unit-atoms  are  called  diatomic  or  bivalent; 
and,  in  a  similar  manner,  there  are  triatomic  or  trivalent, 
tetratomic  or  quadrivalent,  etc.,  elements.  Further,  the  ele- 

*  In  regard  to  the  choice  between  the  three  expressions  given,  it 
may  be  said  that  the  word  valency  seems  to  be  less  objectionable 
than  the  others  which  have  been  used,  because  it  is  the  simplest, 
and,  at  the  same  time,  it  expresses  all  that  it  is  desired  to  express 
with  reference  to  the  property  which  it  designates.  Again,  the 
word  atomicity  has  been  used  in  another  sense,  and  hence  its  use 
might  lead  to  confusion,  some  authors  employing  it  in  its  first  sense, 
others  in  the  new  and  entirely  different  sense.  By  a  monatomic, 
diatomic,  etc.,  element  is  sometimes  meant  an  element  whose  mole- 
cule consists  of  1,  2,  etc. ,  atoms.  As  it  is  necessary  to  have  words 
to  express  this  latter  meaning,  it  seems  desirable  to  leave  the  word 
atomicity  and  its  adjective  derivatives,  monatomic,  diatomic,  etc., 
to  serve  this  purpose,  and  to  adopt  the  expression  valency  with 
the  derivatives  univalent,  bivalent,  trivalent,  etc.,  for  the  purpose 
of  designating  the  property  under  discussion. 


VALENCY.  91 

ments  are  called  respectively  monads,  dyads,  triads,  tetrads, 
pentads,  hexads,  etc.  Accordingly,  of  the  elements  in  the 
brief  table  of  hydrogen  compounds,  given  at  the  beginning 
of  this  chapter  (p.  88),  those  in  the  first  line  are  univalent; 
oxygen,  sulphur,  selenium,  and  tellurium  are  bivalent ; 
nitrogen,  arsenic,  antimony,  and  phosphorus  are  trivalent ; 
and  carbon  and  silicon  are  quadrivalent  toward  hydrogen. 

Distinction  between  Valency  and  Affinity. — The  property 
of  valency  must  not  be  confounded  with  that  of  affinity. 
By  affinity  is  frequently  meant  the  force  that  lies  at  the 
foundation  of  all  chemical  action.  Valency  has  apparently 
no  connection  with  this  force.  An  element  may,  in  general 
terms,  have  a  strong  affinity  for  other  elements,  and  yet  be 
univalent.  Another  may  possess  but  a  weak  affinity,  and 
be  quadrivalent.  Thus,  as  is  sometimes  said,  chlorine  has 
a  strong  affinity  for  hydrogen ;  the  two  combine  with  great 
energy,  yet  they  are  univalent  with  reference  to  each  other. 
While  carbon  does  not  combine  with  chlorine  with  nearly 
as  great  energy  as  hydrogen  does,  it  is,  nevertheless,  quad- 
rivalent toward  hydrogen  ;  its  atom  is  capable  of  holding 
in  combination  4  atoms  of  hydrogen.  These  two  proper- 
ties, valency  and  affinity,  are  possessed  by  every  atom,  and 
exhibit  themselves  every  time  that  atoms  act  upon  each 
other,  the  latter  determining  the  energy  of  the  action,  the 
former  the  complexity  of  the  resulting  molecule. 

Methods  for  Determining  the  Valency  of  the  Elements. — 
The  foundation  upon  which  the  conception  of  valency  rests 
and  the  conception  itself  being  thus  explained,  let  us 
inquire  how  the  valency  of  the  individual  elements  is  deter- 
mined. The  valency  toward  hydrogen  of  those  elements 
which  combine  with  it  may  first  be  determined.  The 
first  general  rules  to  guide  us  in  the  measurements  are  the 
following : — 

1.  Those  elements  which  combine  with  hydrogen  in  the 
proportion  of  1  atom  to  1  atom  are  univalent.     Such,  for 
instance,  are  chlorine,  bromine,  etc. 

2.  Those  elements  which  combine  with  hydrogen  in  the 
proportion  of  1  atom  to  2  atoms  of  hydrogen  are  bivalent. 
Such,  for  instance,  are  oxygen,  sulphur,  etc. 

3.  Those  elements  which  combine  with  hydrogen  in  the 


92       PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

proportion  of  1  atom  to  3  atoms  of  hydrogen  are  trivalent. 
Such,  for  instance,  are  nitrogen,  phosphorus,  etc. 

4.  Those  elements  which  combine  with  hydrogen  in  the 
proportion  of  1  atom  to  4  atoms  of  hydrogen  are  quadriv- 
alent. Such,  for  instance,  are  carbon,  silicon,  etc. 

The  next  question  to  be  answered  is  whether  the  valency 
of  an  element  toward  hydrogen  is  also  its  valency  toward 
other  elements,  as  the  halogens,  oxygen,  etc.  It  requires 
but  a  cursory  examination  to  show  that  this  question 
must  be  answered  in  the  negative,  at  least  for  several  of 
the  elements.  Thus,  while  phosphorus  forms  the  hydro- 
gen compound,  PH3,  and  does  not  combine  with  more 
hydrogen,  it  forms  two  compounds  with  chlorine,  the 
trichloride,  PC13,  and  the  pentachloride,  PC)5.  Sulphur 
forms  with  hydrogen  the  compound  SH2,  in  which  it  is 
bivalent,  and  it  cannot  combine  with  a  larger  proportion  of 
hydrogen.  On  the  other  hand,  it  forms  the  compounds  SC12 
and  SC14  with  chlorine,  and  the  simplest  explanation  of 
these  is  that  in  them  the  sulphur  is  respectively  bivalent 
and  quadrivalent.  Taking  carbon  and  silicon,  however, 
we  find  that  these  elements  have  the  same  valency  toward 
the  halogens  as  toward  hydrogen,  as  is  shown  by  the  com- 
pounds CH4,  SiH4,  CG14,  and  SiClr  As  regards  the  valency 
toward  oxygen  it  can  be  shown  in  the  same  way  that  in 
many  cases  it  is  not  the  same  as  the  valency  toward  hydro- 
gen. The  fact  that  water  has  the  composition  H2O  shows 
that  oxygen  is  bivalent.  In  sodium  oxide,  Na2O,  then,  we 
have  a  compound  analogous  to  water.  In  it  the  oxygen  is 
probably  bivalent.  In  such  compounds  as  calcium  oxide, 
CaO,  magnesium  oxide,  MgO,  in  which  one  atom  of  oxygen 
is  combined  with  one  atom  of  another  element,  the  elements 
are  usually  regarded  as  bivalent.  In  the  same  way  when 
one  atom  of  any  element  holds  in  combination  two  atoms 
of  oxygen  it  is  regarded  as  quadrivalent;  and  when  one 
atom  of  any  element  holds  in  combination  three  atoms  of 
oxygen  it  is  regarded  as  sexivalent  toward  oxygen.  An 
examination  of  the  table  of  oxygen  compounds  given  on 
page  89  will  show  that  the  valency  of  elements  toward 
oxygen  varies  from  1  to  8.  Sodium  is  univalent  toward 
oxygen,  magnesium  is  bivalent,  aluminium  trivalent,  silicon 
quadrivalent,  phosphorus  quinquivalent,  sulphur  sexiva- 
lent, chlorine  septivalent,  and  ruthenium  octivalent.  Other 
views  in  regard  to  these  oxygen  compounds  are  possible,  as 


VALENCY.  93 

will  be  shown  below,  and  these  other  views  have  been  gener- 
ally held.  The  above  is,  however,  the  most  natural  inference 
from  the  facts,  and,  unless  it  can  be  shown  to  be  untenable, 
it  must  have  the  preference  over  more  complicated  views. 

Comparing  now  the  valency  of  a  few  elements  toward 
hydrogen,  with  the  valency  of  the  same  elements  toward 
the  halogens  and  toward  oxygen,  it  will  be  found  that  in 
those  cases  in  which  a  difference  is  observed  the  valency 
toward  hydrogen  is  the  lowest,  and  that  toward  oxygen  the 
highest,  while  that  toward  chlorine  is  intermediate  between 
these.  Thus  sulphur  is  bivalent  toward  hydrogen,  appar- 
ently sexivalent  toward  oxygen,  and  quadrivalent  toward 
chlorine.  Iodine  is  univalent  toward  hydrogen,  apparently 
septivalent  toward  oxygen,  as  shown  by  the  compound  I2O7, 
but  trivalent  toward  chlorine,  and  quinquivalent  toward 
fluorine,  as  shown  by  the  compounds  IC13  and  IF5.  But 
just  as  carbon  and  silicon  are  quadrivalent  toward  hydro- 
gen and  chlorine,  so  they  are  also  quadrivalent  toward 
oxygen,  as  shown  in  the  compounds  CO2  and  SiO2. 

While  it  appears  clear  from  the  above  considerations  that 
the  power  called  valency  is  not  the  same  toward  different 
elements,  it  is  further  true  that  the  valency  of  some  ele- 
ments varies  toward  one  and  the  same  element.  Thus,  it 
has  been  stated  that  phosphorus  is  trivalent  toward  hydro- 
gen and  quinquivalent  toward  chlorine;  but  that  it  is  not 
always  quinquivalent,  or  that  it  does  not  always  appear  to 
be  so,  is  shown  by  the  compound  PC13.  So,  too,  while  sul- 
phur is  bivalent  toward  hydrogen,  quadrivalent  toward 
chlorine,  and  sexivalent  toward  oxygen,  it  also  appears  to 
be  bivalent  toward  chlorine,  and  quadrivalent  toward 
oxygen,  as  shown  by  the  compounds  SCJ2  and  SO2.  Chlo- 
rine, which  is  univalent  toward  hydrogen  and  septivalent 
toward  oxygen,  may  also  act  as  a  univalent  element  toward 
oxygen,  as  is  shown  in  the  compound  C12O. 

Is  Valency  Constant  or  Variable  f — From  what  has  already 
been  said  there  would  appear  to  be  only  one  possible  answer 
to  this  question.  Valency  is  plainly  variable,  if  we  con- 
sider the  composition  of  the  compounds  which  an  element 
forms  as  final  evidence  of  the  valency  of  that  element.  If 
we  consider  valency  as  due  to  something  residing  in  atoms, 
it  is  difficult  to  conceive  of  this  something  as  being  vari- 
able, any  more  than  we  can  conceive  of  the  weight  of 

5* 


94       PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

atoms  as  variable.  How  can  one  and  the  same  atom  have 
at  one  time  the  power  to  combine  with  one  univalent  atom, 
and  at  another  time  three  or  five  times  that  power?  If  it 
has  the  power  to  combine  with  five  univalent  atoms  once, 
it  seems  most  natural  to  suppose  that  it  will  always  have 
that  power.  Considerations  of  this  kind  have  led  to 
various  hypotheses  which  have  for  their  object  the  explana- 
tion of  what  appear  to  be  variations  in  valency,  on  the 
supposition  that  valency  is  a  constant  property  of  atoms. 
There  are  two  cases  to  be  distinguished  between :  (1)  Those 
in  which  the  valency  of  an  element  A  varies  toward  one 
and  the  same  element  B ;  and  (2)  those  in  which  the  valency 
of  an  element  varies  according  to  the  character  of  the  ele- 
ment with  which  it  combines. 

Variation  of  Valency  toward  one  and  the  same  Element. — 
Examples  of  this  kind  of  variation  of  valency  have  already 
been  given.  Phosphorus  appears  to  be  trivalent  and 
quinquivalent  toward  chlorine,  and  also  toward  oxygen, 
as  shown  by  the  compounds  P2O3  and  P2O5;  sulphur  is 
bivalent  and  quadrivalent  toward  chlorine;  and  quad- 
rivalent and  sexivalent  toward  oxygen,  as  shown  by  the 
compounds  SO2  and  SO3.  Carbon  is  bivalent  and  quad- 
rivalent toward  oxygen,  as  is  shown  by  the  compounds 
CO  and  CO2.  Chlorine  is  at  least  univalent  and  trivalent 
toward  oxygen,  as  is  shown  by  the  compounds  C12O  and 
C12O3.  The  chief  hypotheses  which  have  been  put  forward 
to  account  for  variations  of  the  kind  just  mentioned  will 
now  be  briefly  presented. 

Atomic  and  Molecular  Compounds. — One  of  the  first 
attempts  to  explain  variations  in  valency  was  the  hypoth- 
esis that  there  are  two  kinds  of  chemical  compounds, 
the  atomic  and  molecular.  The  former  are  the  only  true 
chemical  compounds,  in  the  sense  in  which  that  ex- 
pression has  hitherto  been  used.  In  these,  the  atoms 
exhibit  all  the  properties  which  have  thus  far  been 
recognized  as  belonging  to  them — including  valency.  By 
virtue  of  these  properties  the  compounds  have  their  exist- 
ence. In  the  molecular  compounds,  on  the  other  hand,  a 
new  force  is  supposed  to  act,  this  force  being  distinct  from 
the  interatomic  force,  and  acting  in  a  peculiar  way  between 
molecules.  The  molecules  are  supposed  to  be  first  formed 


VALENCY. 

by  means  of  chemical  affinity,  etc.,  and,  when  these  have 
been  formed,  all  that  can  be  effected  by  valency  has  been 
effected.  But  now  it  is  further  supposed  that  the  molecules 
thus  formed  have  an  attractive  power  and  combining 
power  of  their  own,  by  virtue  of  which  the  molecular  com- 
pounds are  formed.  The  most  common  examples  of  molec- 
ular compounds  are  salts  containing  water  of  crystalliza- 
tion. These  are  sometimes  considered  to  be  formed  by 
virtue  of  the  attraction  of  the  molecules  of  the  salt  for  the 
molecules  of  the  water.  But,  according  to  the  new  hypoth- 
esis, we  have  further  examples  of  molecular  compounds 
in  phosphorus  chloride,  PC15,  and  in  ammonium  chloride, 
NH4C1.  In  the  former,  a  molecule  of  phosphorus  tri- 
chloride, PCI3,  a  true  atomic  compound,  holds  in  combina- 
tion a  molecule  of  chlorine,  C12,  also  an  atomic  compound. 
In  the  latter,  a  molecule  of  ammonia,  NH3,  an  atomic  com- 
pound, holds  in  combination  a  molecule  of  hydrochloric 
acid,  HC1,  also  an  atomic  compound. 

Foundation  for  the  Distinction  between  Atomic  and  Molec- 
ular Compounds. — Of  course,  in  order  that  such  an 
hypothesis  as  that  under  consideration  should  be  at  all 
permissible,  it  must  be  shown  that  there  are  differences 
between  those  compounds  which  are  called  molecular  and 
those  which  are  called  atomic.  To  a  certain  extent  this 
has  been  supposed  to  be  possible.  In  the  case  of  salts  con- 
taining water  of  crystallization,  there  is,  at  least  generally, 
no  difficulty  in  recognizing  that  the  force  holding  together 
the  salt  and  the  water  is  not  so  strong  as  that  which  holds 
the  constituents  of  the  salt  together,  or  that  which  holds 
the  constituents  of  the  water  together.  It  is  only  neces- 
sary to  heat  the  compounds  gently  in  order  to  overcome 
the  attraction  and  cause  the  breaking  up  of  the  complex 
molecule;  in  some  cases,  indeed,  the  attraction  is  so  weak 
that  it  is  only  necessary  to  expose  the  compound  to  the 
air,  when  the  water  passes  off  in  the  form  of  vapor,  leaving 
the  molecules  of  the  salt  intact.  This  weakness  of  union 
is  then  regarded  as  a  principal  characteristic  of  molecular 
combination. 

Now,  if  the  compounds  above  referred  to,  viz.,  phosphorus 
pentachloride,  PC15,  and  ammonium  chloride,  NH4C1,  are 
examined,  it  will  be  found  that  they  possess  this  character- 
istic. It  has  been  shown,  when  considering  the  cases  of 


96       PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

anomalous  specific  gravities  of  vapor,  that  when  phosphorus 
pentachloride,  PC15,  and  ammonium  chloride,  NH4C1,  are 
heated  to  a  sufficient  degree  to  convert  them  into  vapor, 
they  are  broken  up  into  the  trichloride,  PC13,  and  chlorine, 
CJ2,  and  ammonia,  NH-3,  and  hydrochloric  acid,  HC1,  just  as 
the  crystallized  salts  lose  their  water  of  crystallization  when 
heated.  So,  also,  in  a  number  of  other  cases  it  can  be  shown 
to  be  true  that  compounds,  which  must  be  considered  as 
molecular  in  order  to  explain  their  existence  and  yet  retain 
the  hypothesis  of  constant  valency,  as  above  stated,  break 
down  under  the  influence  of  heat  into  simpler  molecules. 

It  will  thus  be  seen  that  there  is  some  foundation  for 
assuming  the  existence  of  molecular  compounds  as  distinct 
from  atomic  compounds.  But  how  does  this  help  us  ? 

Use  of  the  Distinction. — It  is  plain  that  atomic  compounds 
are  the  only  ones  which  we  can  employ  for  the  purpose  of 
determining  the  valency  of  atoms.  Thus,  the  only  chlorine 
compound  of  phosphorus  that  can  be  employed  for  the 
purpose  of  determining  the  valency  of  phosphorus  is  the 
trichloride.  The  other  compound,  PC15,  the  existence  of 
which  would  seem  to  indicate  that  phosphorus  is  quin- 
quivalent as  well  as  trivalent,  being  according  to  the  new 
hypothesis  a  molecular  compound,  is  formed  independently 
of  the  valency  of  phosphorus  and  does  not  at  all  interfere 
with  the  acceptance  of  the  view  that  valency  is  a  constant 
property.  So,  too,  in  all  similar  cases.  Nitrogen  is  really 
trivalent,  but,  owing  to  the  formation  of  molecular  com- 
pounds, it  appears  oftentimes  to  be  quinquivalent.  By  a 
generous  application  of  this  hypothesis  there  ib  no  difficulty 
in  accounting  for  the  anomalous  cases,  and  the  hypothesis 
of  valency  may  still  be  retained,  unless  it  can  be  shown 
that  there  are  facts,  not  yet  considered,  which  do  not  har- 
monize with  it. 

Difficulties  met  with. — One  difficulty  immediately  presents 
itself.  Although  the  examples  above  given  show  that  there 
are  compounds  which  seem  to  differ  from  true  chemical  com- 
pounds to  a  sufficient  extent  to  justify  the  assumption  that 
they  belong  to  another  class  of  compounds,  still  there  are 
cases  in  which  there  is  no  ground  whatever  for  making  this 
assumption,  and  these,  nevertheless,  show  plainly  that  one  and 
the  same  element  may  be  at  one  time  trivalent,  at  another 


VALENCY.  97 

quinquivalent,  unless  the  above  assumption  is  made  without 
any  experimental  basis  whatever.  The  compound,  phos- 
phorus oxychloride,  POC13,  for  instance,  is  not  decomposed 
when  converted  into  vapor,  and  we  have  just  as  much  right 
to  regard  it  as  a  true  chemical  compound  as  we  have  to  regard 

11111 

the  trichloride,  PC13,  as  such.  But  in  the  oxychloride,  POC13, 
phosphorus  is  apparently  quinquivalent,  while  in  the  trichlo- 
ride, PC13,  it  is  trivalent.  Evidently,  here  the  only  ground 
for  considering  the  oxychloride,  POC13,  a  molecular  com- 
pound is  the  fact  that  its  existence  cannot  be  explained  by  the 
hypothesis  of  constant  valency,  unless  indeed  we  conceive 

OC1 

/ 
it  to  be  represented  by  the  formula  P — Cl,  as  some  have 

\ 

Cl 

done.  This  is  dangerous  reasoning,  and,  if  we  follow  it,  we 
shall  soon  be  in  serious  difficulties.  It  was  seen  above  that 
ammonium  salts  can  be  explained  only  on  the  supposition 
that  the  nitrogen  in  them  is  quinquivalent,  unless  they  are 
assumed  to  be  molecular  compounds.  Plainly,  it  would 
be  next  to  absurd  to  make  this  supposition,  as  they  have 
all  the  characteristics  of  true  chemical  compounds,  and  if 
it  can  be  assumed  that  they  are  only  molecular  compounds, 
in  order  to  suit  our  purposes,  then  we  are  at  liberty  to  make 
the  same  assumption  in  regard  to  almost  any  compound  in 
the  field  of  chemistry. 

Further,  the  fact  that  a  compound  is  easily  decomposed 
by  heat  or  by  any  other  means  is  very  weak  evidence  in 
favor  of  the  view  that  it  is  molecular,  for,  as  is  well  known, 
there  are  many  substances  which  must  be  regarded  as  true 
chemical  compounds,  which  are,  nevertheless,  extremely 
unstable. 

Experiments  showing  that  Nitrogen  may  be  both  Trivalent 
and  Quinquivalent. — Again,  an  experiment  has  been  per- 
formed which  appears  to  prove  that  ammonium  chloride, 
NH4C1,  and  analogous  compounds  of  nitrogen,  are  true 
atomic  compounds.  If  ammonium  chloride,  NH4C1,  is  a 
molecular  compound,  then,  as  was  explained  above,  two 
forces  are  concerned  in  the  formation  of  its  molecule. 

1st.  A  force  holding  together  the  nitrogen  atom  and  three 


98       PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

hydrogen  atoms  forming  the  molecule  NH3 ;  and  the  hydro- 
gen atom  and  chlorine  atom  forming  the  molecule  HC1. 

2d.  A  force  holding  together  the  molecule  NH3  and  the 
molecule  HC1. 

If  these  two  forces  are  distinct  in  character,  the  result- 
ing molecule  may  be  represented  by  the  formula  (NH3-f 
HC1).  Suppose  now  we  add  together  two  other  mole- 
cules, such  that,  taken  together,  their  constituent  atoms 
are  the  same  in  number  and  quantity  as  those  contained 
in  the  compound  (NH3-}-HCl).  Then  the  resulting  com- 
pound ought  not  to  be  identical  with  that  obtained  in  the 
former  case.  If  these  new  molecules  are,  for  instance, 
NH2C1  and  H2,  then  the  compound  will  be  (NH2C1+HH), 
and  this  should  not  be  identical  with  (NH3-{-HCl),  although 
its  composition  is  the  same. 

This  method  of  investigation  has  been  applied  to  the 
study  of  the  problem  under  consideration,  not,  indeed,  with 
the  molecules  employed  in  the  above  explanation,  but  with 
others  analogous  to  them.  Instead  of  ammonia,  NH3,  the 
analogous  compound,  N(CH3)3,  was  taken,  and  this  was 
united  with  ethyl  iodide,  C2H5I.  Thus,  a  compound  was 
obtained  which,  if  it  is  molecular,  should  be  represented 
by  the  formula  (N(CH3)3-j-C2H5I).  Again,  the  compound, 
N(CH3)2C2H5,  was  united  with  methyl  iodide,  CH3I,  yield- 
ing a  compound  which  should  be  represented  by  the 
formula  (N(CH8)2C2H6+CH3I).  Now  these  two  new  com- 
pounds ought  not  to  be  identical,  if  they  are  molecular 
and  not  atomic.  On  comparing  them,  however,  they  were 
found  to  be  in  every  respect  identical. 

From  this  experiment  it  is  concluded  that  the  compounds 
studied  are  atomic  compounds,  and  that  in  them  nitrogen 
is  quinquivalent.  Each  group  (CH8),  (C2H5),  and  the 
element,  I,  being  held  by  the  same  kind  of  force,  the  iden- 
tity of  the  resulting  compounds  is  readily  understood. 

As  was  stated  in  a  previous  chapter,  the  compound, 
PC15,  can  be  converted  into  vapor  under  certain  circum 
stances,  viz.,  in  the  presence  of. the  vapor  of  the  trichloride, 
PC13.  From  this  it  is  concluded  that  the  compound,  PC16, 
is  a  true  chemical  or  atomic  compound ;  and,  hence, 
that  the  phosphorus  atom  contained  in  it  is  quinquivalent. 
And,  further,  ammonium  chloride  can  be  converted  into 
vapor  with  but  little  decomposition  in  the  presence  of 
ammonia  or  of  hydrochloric  acid. 


VALENCY.  99 

The  Distinction  between  Atomic  and  Molecular  Com- 
pounds unnecessary  as  far  as  the  Hypothesis  of  Valency  is 
concerned. — It  appears,  therefore,  that  nitrogen  and  phos- 
phorus act  in  some  compounds  as  trivalent,  in  other 
compounds  as  quinquivalent,  elements.  If  this  is  ac- 
knowledged, however,  then  there  is  no  necessity  for  assum- 
ing the  existence  of  molecular  compounds  for  the  purpose 
of  explaining  anomalies  in  the  valency  of  elements.  It  is 
possible  that  in  some  of  the  so-called  double  compounds, 
in  which  two  or  more  salts  appear  to  be  combined  with 
each  other,  as  well  as  in  the  salts  containing  water  of 
crystallization,  we  have  examples  of  true  molecular  com- 
pounds, in  the  sense  in  which  this  expression  has  been 
used  in  the  present  article ;  but  it  is  evident  that,  as  soon 
as  the  possibility  of  one  and  the  same  element  being  either 
univalent  or  bivalent  is  admitted,  it  is  no  longer  necessary 
to  assume  the  existence  of  these  molecular  compounds.  At 
present  the  hypothesis  of  molecular  compounds  does  not 
play  an  important  part  in  dealing  with  the  phenomena  of 
valency.  It  seems  not  improbable  that  when  the  subject 
of  valency  is  thoroughly  understood  it  will  be  found  com- 
petent to  explain  all  compounds  that  are  made  up  of  con- 
stituents combined  in  definite  proportions. 

Saturated  and  Unsaturated  Compounds. — As  soon  as  the 
quinquivalence  of  nitrogen  was  established  a  new  explana- 
tion was  offered  concerning  the  nature  of  nitrogen  com- 
pounds. Only  those  compounds  in  which  the  nitrogen 
is  quinquivalent  were  looked  upon  as  complete.  Those 
in  which  the  nitrogen  is  trivalent  were  looked  upon  as 
incomplete.  For  the  expressions  complete  and  incomplete, 
the  expressions  saturated  and  unsaturated  were  employed. 
The  atom  of  nitrogen  having  the  power  to  hold  in  com- 
bination five  univalent  elements  or  groups  is  saturated 
when  all  of  its  powers  are  employed,  as  in  the  compound 
NH4C1 ;  it  is  unsaturated  when  only  a  part  of  its  powers 
are  employed,  as  in  the  compound  NH3.  The  power  of 
the  nitrogen  atom  was  expressed  by  saying  that  it  pos- 
sesses five  affinities  or  bonds.  In  the  saturated  compound 
all  of  these  affinities  are  employed,  whereas  in  the  unsat- 
urated compound  only  a  part  of  them  are  employed. 

Maximum  Valency  and  Apparent  Valency. — According 
to  the  hypothesis  of  saturated  and  unsaturated  compounds, 


100    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

then,  each  element  is  conceived  to  have  a  certain  maximum 
valency  toward  every  other  element.  But  an  element  may 
not  always  act  with  its  maximum  valency,  and  may  appear 
to  have  a  lower  valency  than  the  maximum.  The  maximum 
valency  of  phosphorus  toward  chlorine  is  that  shown  in 
the  pentachloride.  In  the  trichloride  its  apparent  valency 
is  three;  nevertheless,  the  phosphorus  is  quinquivalent 
toward  chlorine,  as  is  shown  by  bringing  chlorine  in  con- 
tact with  the  unsaturated  compound,  when  it  becomes 
saturated  to  form  the  pentachloride.  So  also  carbon  is 
regarded  as  quadrivalent  toward  oxygen,  but  its  apparent 
valency  in  carbon  monoxide,  CO,  is  two. 

Maximum  Valency  is  dependent  upon  Conditions. — As 
regards  the  minimum  valency  of  an  element  no  absolute 
statement  can  be  made.  When  the  statement  is  made  that 
phosphorus  is  quinquivalent,  the  words,  at  ordinary  tem- 
peratures, must  be  added  if  we  are  to  restrict  our  statement 
to  what  we  know.  It  has  already  been  shown  that  at  ele- 
vated temperatures  phosphorus  pentachloride  is  decomposed 
into  the  trichloride  and  chlorine.  Thus  it  is  clear  that  at 
this  elevated  temperature  phosphorus  is  trivalent  toward 
chlorine.  So,  too,  while  nitrogen  is  quinquivalent  at  the 
ordinary  temperature  toward  hydrogen  and  chlorine  to- 
gether, it  becomes  trivalent  at  a  higher  temperature.  In 
general,  the  complexity  of  compounds  becomes  less  as  their 
temperature  is  elevated ;  or,  in  other  words,  the  valency  of 
the  elements  decreases  with  increase  of  temperature. 

Are  all  the  Bonds  of  an  Element  of  the  same  order  f — 
This  question  is  suggested  by  the  easy  decomposition  of 
such  compounds  as  phosphorus  pentachloride;  ammonium 
chloride,  etc.,  when  heated.  To  account  for  the  facts  it 
has  been  assumed  that  of  the  five  bonds  or  affinities  of  the 
nitrogen  atom  and  of  the  phosphorus  atom  two  are  weaker 
than  the  other  three.  Hence,  in  a  saturated  nitrogen  com- 
pound two  atoms  or  groups  have  been  supposed  to  be  held 
less  firmly  in  combination  than  the  other  three,  and  they 
are  given  off  more  readily.  The  experiment  described 
above,  however,  which  proved  the  identity  of  the  com- 
pounds [N(CHS)3+  C2H5I]  and  [N(CH3)2(C2H5)  +  CH8I], 
also  appears  to  show  that  the  affinities  of  the  nitrogen 
atom  are  all  of  the  same  kind,  and  hence  it  cannot  be 


VALENCY. 

admitted  that  two  of  the  affinities  are  weaker  than  the  other 
three.  While,  further,  phosphorus  pentachloride,  PC15,  is 
readily  decomposed  by  heat,  in  accordance  with  the  suppo- 
sition that  two  of  the  affinities  of  the  phosphorus  atom  are 
weaker  than  the  other  three,  yet,  on  the  other  hand,  the 
compound,  POC13,  gives  no  evidence  of  this  difference  be- 
tween the  affinities.  It  can  also  easily  be  shown  that  it  is 
not  necessary  to  assume  this  difference  in  order  to  explain 
the  decomposition  of  phosphorus  pentachloride,  PC15,  and 
ammonium  chloride,  NH^Cl.  It  may,  for  instance,  be  sup- 
posed that  the  five  affinities  of  the  nitrogen  atom,  or  the 
phosphorus  atom,  are  all  exactly  equal  in  power  at  the 
outset.  Should  three  of  these  affinities,  however,  become 
saturated,  it  seems  possible  that  the  presence  of  the  satu- 
rating atoms  or  groups  may  influence  the  power  of  the 
remaining  unsaturated  affinities  in  such  a  way  as  to  make 
them  weaker  than  they  were  at  first;  or  it  may  be  assumed 
that  in  the  saturated  compound  each  atom  of  chlorine  is 
held  by  the  phosphorus  atom  with  exactly  the  same  force, 
but  that  this  force  is  less  than  that  exerted  between  the 
phosphorus  and  each  atom  of  chlorine  in  the  trichloride. 

It  seems  rational  to  suppose  that  in  phosphorus  trichlo- 
ride, for  example,  all  three  of  the  chlorine  atoms  are  in- 
fluenced in  exactly  the  same  way  by  the  phosphorus,  and 
that  the  whole  atom  of  phosphorus  is  brought  into  play. 
Further,  that  under  certain  circumstances  the  same  power 
of  the  phosphorus  may  be  distributed  uniformly  among 
five  chlorine  atoms,  but  that  then  the  compound  is  less 
stable  than  the  simpler  one.  On  heating  the  less  stable 
compound  it  loses  two  chlorine  atoms,  and  the  more  stable 
one  is  formed. 

Self  saturation  of  Atoms. — To  account  for  the  existence 
of  unsaturated  compounds  it  has  been  suggested  that  two 
affinities  of  the  same  atom  might  in  some  way  act  upon 
each  other,  causing  saturation.  The  compound  in  which 
such  a  combination  exists  is  then  a  complete  compound  not 
possessing  free  affinities.  In  such  compounds,  however, 
the  mutual  saturation  of  like  affinities  can  easily  be  over- 
come, and  then  other  atoms  can  be  held  in  combination. 
This  explanation  appears  probable  in  the  light  of  the  fact 
that  the  number  of  latent  affinities  in  unsaturated  com- 
pounds is  always,  with  very  few  exceptions,  an  even 


102    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

number.  This  would  necessarily  be  the  case  if  the  above 
assumption  were  true.  There  are,  however,  undoubted 
exceptions  to  this  rule,  among  the  most  marked  being 
nitric  oxide,  NO,  and  tungsten  pentachloride,  WCJ5,  tung- 
sten having  a  maximum  valency  of  six  toward  chlorine. 
On  the  other  hand,  phosphorus  and  nitrogen  are  trivalent 
or  quinquivalent,  sulphur  bivalent,  quadrivalent,  or  sexiv- 
alent,  the  halogens  univalent,  trivalent,  quinquivalent,  or 
septivalent.  Notwithstanding  the  exceptions,  this  rule  of 
change  is  suggestive  and  needs  explanation. 

Single,  Double,  and  Triple  Linkage  of  Atoms  of  the  same 
Kind. — Another  explanation  of  apparent  variation  in 
valency  applicable  to  a  number  of  cases  is  that  atoms  of 
the  same  kind  may  partly  saturate  each  other,  somewhat 
as  they  are  supposed  wholly  to  saturate  each  other  in  the 
molecules  of  polyvalent  elements.  For  example,  the  mole- 
cule of  oxygen  is  regarded  as  made  up  of  two  bivalent 
atoms,  and  is  represented  by  the  symbol  O— O.  Here 
the  atoms  are  regarded  as  united  by  double  linkage,  a 
condition  of  things  which  is  also  believed  to  exist  in  such 
compounds  as  the  oxides  of  calcium,  magnesium,  etc., 
Ca=O,  Mg=O.  If,  however,  these  atoms  are  regarded 
as  united  by  single  linkage  as  oxygen  and  hydrogen  are 
united  in  water  H — O — H,  then  the  double  atom  of  oxy- 
gen, instead  of  being  saturated,  will  itself  be  bivalent,  as 
it  is  supposed  to  be  in  hydrogen  peroxide,  H— O — O — H. 
This  explanation  is  commonly  accepted  for  the  compounds 
of  copper  and  mercury.  These  elements  are  bivalent,  as 
shown  by  the  compounds  CuCl2,  HgCJ2,  CuO,  HgO,  etc. 
Nevertheless,  as  is  well  known,  in  the  so-called  cuprous 
and  mercurous  compounds  they  are  apparently  univalent. 
It  is  suggested,  however,  that  in  these  latter  compounds  a 
condition  of  things  exists  like  that  above  referred  to  as 
probably  existing  in  hydrogen  peroxide.  If  in  the  cuprous 
compounds  two  copper  atoms  are  in  combination  by  one 
bond,  the  double  atom  is  bivalent,  or  the  single  atom 
would  appear  to  be  univalent,  and  the  same  remark  applies 
to  the  mercurous  compounds.  This  view  is  expressed  in 
the  formulas : — 

CUv 

Cl— Cu— Cu— Cl,    cuprous    chloride;     |  V),   cuprous 

Cu' 


V.'  -  M    Y 

ur 


UNIVERSITY 

^LCALJFQ^^ 

VALENCY.  103 


gx 

oxide  ;    Cl  —  Hg  —  Hg  —  Cl,    mercurous   chloride  ;         X), 

Hg/ 
mercurous  oxide. 

But  just  as  atoms  of  the  same  kind  must  in  many  cases 
be  regarded  as  held  in  combination  by  single  linkage,  so 
too,  in  many  cases,  the  assumption  that  atoms  of  the  same 
kind  are  united  by  double  or  by  triple  linkage,  affords  a 
simple  explanation  of  variations  in  valency.  Examples 
are  furnished  by  the  three  chlorides  of  carbon,  C2C16,  C2C14, 
and  C2C12,  and  the  three  hydrocarbons,  C2H6,  C2H4,  and 
C2H2.  In  all  these  carbon  is  regarded  as  quadrivalent; 
but,  while  in  the  chloride,  C2C16,  the  carbon  atoms  are 
believed  to  be  united  in  the  simplest  way,  called  single 
linkage,  in  the  chloride,  C2C14,  they  are  believed  to  be 
united  by  double  linkage,  and  in  the  chloride,  C2C12,  by 
triple  linkage.  These  ideas  are  expressed  in  the  formu- 
las: — 

Cl  Cl  Cl  Cl 


II  II 

l_C— C— Cl;  O= C; 


Cl— C— C— Cl;  0= C;  Cl— feC— Cl. 

II  II 

Cl  Cl  Cl  Cl 

If  the  question  is  asked  what  is  meant  by  these  double 
and  triple  lines,  it  can  only  be  answered  that  they  are 
intended  to  express  the  idea  that  in  the  case  of  the  doubly 
linked  atoms  the  condition  of  single  linkage  is  probably 
repeated  twice,  and  that  in  the  case  of  triply  linked  atoms 
this  condition  is  repeated  three  times.  In  regard  to  the 
nature  of  single  linkage  itself  we  have  no  clear  conception. 
As  regards  experimental  evidence  in  favor  of  these  views 
it  must  be  acknowledged  that  there  is  very  little,  and  that 
is  furnished  by  a  study  of  the  compounds  of  carbon.  It 
will  be  presented  when  these  compounds  are  taken  up. 

It  will  be  noticed  that  the  above  method  of  explaining 
apparent  variations  in  the  valency  of  double  atoms  implies 
that  these  variations  take  place  by  pairs.  That  is  to  say, 
the  valency  of  a  double  atom  may,  according  to  this,  be 
two,  four,  six,  eight,  etc.,  but  such  a  double  atom  would 
never  have  a  valency  of  one,  three,  five,  etc.  A  similar 
explanation  can  be  offered  for  cases  in  which  groups  of 
atoms  have  a  valency  of  one,  three,  five,  etc.,  but  cases  of 
this  kind  are  not  known,  and  the  explanation  is  therefore 


104    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

superfluous.  This  change  of  valency  by  pairs  in  the  case 
of  groups  of  atoms  suggests  that  possibly  some  similar 
cause  may  operate  in  the  case  of  the  variations  in  valency 
of  what  we  call  single  atoms.  The  connection  existing 
between  the  properties  of  the  elements  and  their  atomic 
weights,  which  is  clearly  recognized  in  the  periodic  law, 
suggests  that  what  we  call  elements  are  in  reality  not  ele- 
ments, but  compounds  of  the  true  elements.  If  this  be 
true,  and  probably  very  few  doubt  it,  then  it  follows  that 
what  we  call  atoms  are  not  atoms,  but  groups  of  atoms. 
The  change  in  the  valency  may  possibly  be  due  to  changes 
in  the  relations  of  the  atomicules  of  which  the  atoms  them- 
selves may  be  made  up.  It  must  be  acknowledged  that 
at  present  there  is  no  evidence  in  favor  of  this  view.  It  is 
simply  an  interesting  speculation. 

Relative  Valeney. — It  will  be  clear  from  what  has  been 
said  that  the  variations  in  valency  are  not  always  supposed 
to  be  due  to  the  same  cause.  That  it  varies  with  the  tem- 
perature cannot  be  questioned.  Why,  we  do  not  know. 
Again,  that  it  apparently  varies  in  consequence  of  the 
union  of  atoms  of  the  same  kind  in  different  ways  is  a 
very  plausible  hypothesis,  which  satisfactorily  explains  a 
number  of  cases.  The  suggestion  that  some  compounds 
are  saturated  and  others  unsaturated  is  in  itself  not  an 
attempt  at  an  explanation,  but  is  merely  a  restatement  of 
the  fact  that  demands  explanation.  There  is  no  well- 
established  hypothesis  to  account  for  those  cases  in  which 
the  valency  of  an  atom  varies  with  reference  to  some  other 
atom,  as  in  the  case  of  phosphorus  and  chlorine,  carbon  and 
oxygen,  etc.  The  suggestion  that  atoms  consist  of  groups 
of  atomicules,  if  it  could  be  shown  to  be  well  founded, 
would  bring  the  cases  referred  to  within  the  applications 
of  the  hypothesis  made  use  of  to  account  for  variations  in 
the  valency  of  groups  of  atoms.  In  the  early  part  of  this 
chapter  it  was  pointed  out  that  the  valency  of  an  element 
varies  according  to  the  character  of  the  elements  with  which 
it  combines.  In  other  words,  valency  is  a  relative  property. 
The  valency  of  an  element  may  be  constant  toward  some 
elements  and  variable  toward  others.  Thus,  the  valency 
of  the  members  of  the  chlorine  family  toward  hydrogen 
and  toward  most  of  the  so-called  metals  is  constant,  while 


VALENCY.  105 

toward  oxygen,  and  toward  hydrogen  and  oxygen  together, 
it  apparently  varies  between  one  and  seven.  The  valency 
of  sulphur  toward  hydrogen  is  constant,  but  toward  oxygen, 
and  toward  hydrogen  and  oxygen,  it  varies  from  four  to 
six,  and  toward  chlorine  from  two  to  four. 

At  present  no  satisfactory  explanation  can  be  offered 
of  the  fact  that  valency  is  relative.  It  is  clear  that  the 
hypothesis  adopted  to  account  for  variations  in  valency 
toward  one  and  the  same  element  will  not  explain  why 
sulphur  should  be  bivalent  toward  hydrogen  and  sexiva- 
lent  toward  oxygen.  It  is  true  a  similar  hypothesis  has 
been  put  forward  to  explain  some  cases  of  apparently 
variable  valency.  Thus,  it  has  been  suggested  and  long 
taught  that  chlorine  is  univalent  toward  oxygen  as  well 
as  toward  hydrogen,  and  that  in  the  oxides  and  oxygen 
acids  of  chlorine  the  atoms  of  oxygen  are  combined  in 
what  are  called  chains.  Thus,  instead  of  assuming  that 

,O 

chlorine  is  trivalent  in  the  compound  C12O3  or        ^)O 


it  was  assumed  to  be  univalent  and  the  compound  repre- 
sented thus,  Cl — 0— O— O— Cl.  The  only  reason  for  this 
was  that  valency  was  regarded  as  necessarily  constant ; 
and,  as  it  is  univalent  in  hydrochloric  acid,  it  must, 
therefore,  be  univalent  in  all  other  compounds.  In  the 
same  way  perchloric  acid  was  considered  as  made  up  as 
represented  in  the  formula  Cl— O— O— O— O— H.  The 
chief  objection  to  this  view  is  that  it  is  founded  upon 
the  hypothesis  of  constant  valency,  which  is  not  tenable. 
Another  objection  to  it  is  that,  so  far  as  can  be  determined, 
compounds  in  which  oxygen  atoms  are  linked  together  are 
unstable,  while  the  chlorates  and  perchlorates  are  relatively 
stable  substances,  and  the  stability  of  the  oxygen  com- 
pounds of  chlorine  is  greater  the  greater  the  quantity  of 
oxygen  in  them.  It  will  also  be  shown  further  on  that 
many  compounds  of  iodine  can  be  understood  only  on  the 
supposition  that  they  contain  septivalent  iodine,  and  that 
all  the  oxygen  acids  of  the  halogens  are  easily  explained 
on  the  supposition  that  in  them  the  halogens  are  either  univ- 
alent, trivalent,  quinquivalent,  or  septivalent. 


106    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

Periodic  Variations  in  Valency. — The  valency  of  the  ele- 
ments acquires  additional  interest  when  examined  in  the 
light  of  the  periodic  law.  It  will  be  seen  that  the  valency 
varies  with  the  position  of  the  elements  in  the  periodic 
system.  This  is  shown  to  some  extent  in  the  tables  on 
pages  83,  84,  and  86.  Referring  to  that  on  page  83  it 
appears  that  from  Group  I.  to  Group  VIII.  there  is  a  regu- 
lar increase  in  the  proportion  of  oxygen  contained  in  the 
oxides  represented  by  the  formulas — 

R2O    R2O2    R2O3    R2O4    RA    RA    R2O7    R2O8 

(RO)  (R02)  (R03)  (R04) 

As  has  already  been  pointed  out,  it  is  possible  that  in 
some  of  these  oxides  oxygen  may  be  linked  to  oxygen,  in 
which  case  the  oxygen  valency  would  not  be  as  great  as 
appears  from  the  formula.  Considering,  on  the  other  hand, 
the  regularity  of  the  increase  from  one  end  of  the  series  to 
the  other,  the  view  that  this  regular  increase  in  complexity 
is  due  to  increased  valency  for  oxygen  appears  to  be  the 
most  rational.  It  may  be  said  in  general,  then,  that  the 
oxygen  valency  of  the  elements  increases  regularly  from 
one  to  eight  from  Group  I.  to  Group  VIII.  The  members 
of  Groups  I.  to  III.  cannot  be  spoken  of  as  having  any 
hydrogen  valency,  or,  perhaps  better,  our  knowledge  of 
the  hydrogen  compounds  of  these  elements  is  so  imperfect 
that  no  definite  statement  can  be  made  concerning  the 
hydrogen  valency.  Beginning  with  Group  IV.  the  highest 
hydrogen  valency  is  exhibited,  and  from  this  to  Group 
VII.  there  is  a  regular  decrease  in  the  hydrogen  valency 
from  four  to  one.  Further,  the  valency  of  an  element 
toward  hydrogen  is  constant. 

Similar  regularities  are  observed  in  the  chlorine  valency, 
which,  like  the  oxygen  valency,  varies  in  one  and  the  same 
element.  The  maximum  chlorine  valency  is  six,  as  in  the 
compound  tungsten  hexachloride,  WC16.  There  is  a  regular 
increase  in  the  chlorine  valency  from  one  in  Group  I.  to  six 
in  some  members  of  Group  VI.,  and  then  a  falling  off  when 
Group  VII.  is  reached.  There  are,  further,  some  irregu- 
larities presented  by  the  chlorides  which  need  explanation, 
as,  for  example,  the  easy  transformation  of  tungsten  hexa- 
chloride into  the  pentachloride.  Another  peculiarity  met 
with  among  the  chlorine  compounds  is  that  some  of  the 
members  of  Group  III.,  as,  for  example,  aluminium,  form 


VALENCY.  107 

chlorides  of  the  formula  M2C16,  according  to  which  they  ap- 
pear to  be  quadrivalent,  while  their  position  in  the  system 
and  the  composition  of  many  of  their  compounds  indicate 
that  they  are  trivaleut.  As  regards  these  cases,  however, 
it  has  been  suggested  that  in  them  the  chlorine  may  be  the 
linking  element.  If  it  be  assumed  that  a  double  chlorine 
atom  (C12)  can  take  the  place  of  one  oxygen  atom,  then 
not  only  the  compounds  like  A12CJ6,  but  all  the  so-called 
double  chlorides  can  be  easily  explained.  This  view  has 
been  fully  discussed  in  articles  by  the  author  of  this 
book.* 

The  hydroxides  can  all  be  explained  most  readily  by 
assuming  that  the  valency  of  the  elements  toward  oxygen 
and  hydroxyl  (OH)  increases  regularly  from  one  to  seven 
from  Group  I.  to  Group  VII.  It  will  be  shown  further  on 
upon  what  grounds  the  hydroxides  are  regarded  as  con- 
taining the  univalent  group  hydroxyl  (  —  O  —  H).  The 
univalent  metals  form  hydroxides  of  the  general  formula 
M(OH),  the  bivalent  metals  form  hydroxides  of  the  general 
formula  M(OH)2,  etc.  Now,  if  the  table  of  hydroxides  on 
p.  81  is  examined,  it  will  be  seen  that  up  to  Group  IV. 
there  is  a  regular  increase  in  the  number  of  hydroxyl 
groups  held  in  combination.  The  hydroxides  of  Group 
IV.  show  a  marked  tendency  to  give  up  the  elements  of 
water,  and  thus  to  form  compounds  with  a  smaller  number 
of  hydroxyls.  Thus  the  hydroxide  of  carbon  correspond- 
ing to  the  maximum  oxygen  valency  would  have  the  formula 
C(OH)4;  but  this  compound  cannot  exist.  It  loses  water 
and  passes  back  to  the  form  CO(OH)2,  and  even  this  is 
extremely  unstable,  breaking  down  at  the  slightest  eleva- 
tion of  temperature  into  water  and  carbon  dioxide.  The 
maximum  hydroxide  of  silicon  can  be  prepared,  but  it 
changes  to  the  acid  SiO(OH)2,  and  this  further  to  SiO2, 
though  these  changes  take  place  much  less  readily  than  in 
the  case  of  carbon.  The  simplest  interpretation  of  the 
changes  referred  to  is  that  the  valency  of  the  elements  is 
not  changed.  Representing  the  maximum  hydroxide  of 
OH  OH 


/  OTT 

silicon    by    Si  1  or    Si  <QJJ>    the    simplest    view 


OH  OH 

*  See  American  Chemical  Journal,  Vols.  XI.,  XIV. 


108     PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 
is  that,  when   it   loses   a  molecule  of  water,  it  becomes 

,/° 

Si — O — H,  in  which  the  silicon  is  still  quadrivalent.     In 

XO— H 

the  same  way  the  maximum  hydroxides  of  Group  V.  would 
have  the  formula  M(OH)5,  but  these  do  not  exist.  They 
lose  water  and  form  compounds  of  the  general  formula 
MO(OH)3  or  M02(OH),  as,  for  example,  phosphoric  acid, 
PO(OH)3  and  nitric  acid  NO2(OH).  But,  just  as  the 
oxygen  valency  of  the  members  is  variable,  being  in  some 
compounds  three  and  in  others  five,  so,  too,  the  hydroxyl 
valency  varies.  There  are  compounds,  for  example,  de- 
rived from  the  hydroxides  M(OH)3.  Phosphorous  acid 
may  be  the  phosphorus  compound  of  this  formula;  and 
nitrous  acid  appears  to  be  derived  from  the  hydroxide 
N(OH)3  by  loss  of  water,  much  in  the  same  way  that  nitric 
acid  is  derived  from  the  hydroxide  N(OH)6.  It  is  accord- 
ingly represented  by  the  formula  NO(OH). 

In  Group  Vlfl  the  maximum  hydroxides  have  the  gen- 
eral formula  M(OH)6,  but  these  generally  break  down, 
losing  two  molecules  of  water,  and  forming  compounds 
MO2(OH)2,  of  which  sulphuric  acid,  SO2(OH)2,  and  chromic 
acid,  CrO2(OH)2,  are  examples.  In  these  compounds  the 
sulphur  and  chromium  are  regarded  as  sexivalent,  and  this 

O.        0-H         O.          0-H 
view  is  thus  expressed :      \S(  and      ^Cr^ 

(K    XO— H         O^     XO— H 

Selenious  acid  and  similar  acids  are  regarded  as  derived 
in  the  same  way  from  quadrivalent  hydroxides  M(OH)4. 
Thus  selenious  acid  is  represented  by  the  formula 
SeO(OH)2. 

In  Group  VII.,  finally,  the  maximum  hydroxides  have 
the  formula  M(OH)7,  but  these  generally  break  down  to 


the  form  MO3(OH)  or  M — ^     ;  examples  are  perchloric 


and  periodic  acids.  Other  compounds  are  derived  from 
the  hydroxides  M(OH)5,  M(OH)3,  and  M(OH).  The  first 
two,  like  the  hydroxide  M(OH)7,  break  down  to  a  hy- 


VALENCY.  109 

droxide  containing  but  one  hydroxyl.  Examples  are 
chloric  acid,  CIO,  (OH),  and  chlorous  acid,  CIO  (OH). 
Thus  it  will  be  observed  that  the  maximum  hydroxides  of 
the  members  of  Groups  V .,  VI.,  and  VII.  do  not  exist,  or 
are  very  unstable  compounds,  but  that  they  break  down 
by  loss  of  water  generally  forming  compounds  in  which 
the  number  of  hydroxyl  groups  corresponds  to  the  hydrogen 
valency  of  the  members  of  the  group. 

Finally,  the  valency  of  an  element  is  greater  towards 
similar  elements  than  towards  those  of  a  different  kind. 
This  is  shown  most  strikingly  in  the  case  of  the  members 
of  Group  VII.  Towards  oxygen,  which  they  resemble 
in  many  respects,  their  valency  is  high ;  towards  hydrogen 
and  the  alkali  metals  the  valency  is  low.  The  same  is 
true,  though  in  a  less  marked  way,  of  the  members  of 
Groups  VI.  and  V.,  while  the  members  of  Group  IV. 
have  the  same  valency  towards  hydrogen,  oxygen,  and 
chlorine. 

Classification  of  the  Elements  with  Reference  to  their 
Valency. — As  the  valency  of  the  elements  varies  in  many 
cases  both  towards  one  and  the  same  element,  and  towards 
different  elements,  it  is  plain  that  any  attempt  to  classify 
the  elements  according  to  the  valency,  except  in  some 
very  broad  way,  must  be  unsuccessful.  Nor  is  there  much, 
if  any,  value  in  such  attempts  at  present.  We  must  first 
learn  more  about  the  laws  governing  the  variations ;  and 
we  shall  acquire  the  desired  knowledge  not  by  assuming 
that  because  an  element  has  a  certain  valency  in  one  com- 
pound it  must  therefore  have  it  in  all  others,  and  then 
turning  and  twisting  the  facts  so  as  to  avoid  giving  up 
this  unwarranted  assumption;  but  by  determining  inde- 
pendently in  as  many  cases  as  possible  what  valency  the 
elements  actually  exhibit,  and  then  seeing  what  general 
conclusions  can  be  drawn. 

Application  of  the  Views  concerning  Valency  to  the  Study 
of  Chemical  Compounds. — Up  to  the  present  the  chief  value 
of  the  views  concerning  valency  has  been  in  connection 
with  the  study  of  the  constitution  of  the  compounds  of 
carbon,  and  particularly  for  the  purpose  of  expressing  the 
connections  between  carbon,  oxygen,  and  hydrogen.  The 
application  to  the  study  of  the  constitution  of  the  so-called 

6 


HO    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

inorganic  compounds  has  not  been  as  successful,  mainly 
for  the  reason  that  these  compounds  do  not  undergo  trans- 
formations as  readily  as  those  of  carbon,  and  therefore  not 
as  much  is  known  about  them.  On  the  other  hand,  the 
constitution  of  most  of  the  familiar  inorganic  compounds 
is  comparatively  simple,  and  the  determination  of  their 
constitution  is  also  comparatively  simple. 


CONSTITUTION.  HI 


CHAPTER  VIII. 

CONSTITUTION  OR  STRUCTURE  OF  CHEMICAL  COMPOUNDS. 
DEFINITION  OP  CONSTITUTION,  ETC. 

Definition,  etc. — If  all  chemical  compounds  could  be  con- 
verted into  vapor  without  suffering  decomposition,  and  the 
truth  of  Avogadro's  hypothesis  were  established  beyond 
reasonable  doubt,  it  would  be  possible  to  determine  the 
molecular  formulas  of  these  compounds.  The  steps  in- 
volved in  determining  the  formula  of  a  compound  are 
these : — 

1.  The  compound   must  be  analyzed;   the  percentages 
of  its  constituents  must  be  determined.     This  involves  no 
hypothesis.     If,  in  expressing  the  results  of  the  analysis, 
we  say  the   compound    contains  a  certain   percentage   of 
carbon,  a  certain  percentage  of  oxygen,  and  a  certain  per- 
centage of  hydrogen,  we  simply  state  facts. 

2.  If,  however,   we   express  the  results  by  a  chemical 
formula,  we  then   make  use  of  some  hypothesis.     At  the 
present  day  the  hypotheses  involved  in  the  simplest  chem- 
ical formulas,  such   as  HC1,  H2O,  H3N,  etc.,  are  (1)  the 
atomic   hypothesis,    and   (2)    Avogadro's   hypothesis.     In 
writing  these  formulas  we  mean  to  express  the  composition 
of  a   molecule   of  each   compound.     In   many  cases  the 
molecular  weight  cannot  be  determined.     Thus,  take  sodium 
sulphate.    We  write  the  formula  Na2SO4  for  the  compound, 
but  we  have  no  means  of  judging  whether  this  formula  ex- 
presses the  true  molecular  weight  of  the  compound  or  not. 
It  expresses  the  percentage  composition,  and,  if  we  accept 
the  atomic  weights  as  determined  by  the  rules  of  Avogadro 
and  of  Dulong  and  Petit,  the  formula  given  is  the  simplest 
one  possible.     This  is  the  only  reason  for  adopting  it.     As 
far  as  experimental  evidence  is  concerned,  the  formulas 
Na4S2O8  and  Na^Ou  are  fully  as  satisfactory,  and,  indeed, 
there  are  some  facts  known  which  seem  to  indicate  that 
these  more  complex  formulas  are  really  nearer  the  truth 
than  the  simple  one  now  in  use,     This,  however,  is  not  a 


112    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

matter  of  much  importance  in  the  present  stage  of  chem- 
istry. For  most  purposes  the  formula  Na2SO4  is  quite  satis- 
factory. 

The  formulas  which  simply  express  the  percentage  compo- 
sition of  molecules  are  commonly  called  empirical  formulas. 
Can  anything  else  be  fairly  represented  by  a  chemical  for- 
mula? It  will  be  observed  that  the  empirical  formulas 
have  nothing  whatever,  to  do  with  the  conduct  of  the  com- 
pounds they  represent.  On  studying  the  action  of  chemical 
compounds  upon  one  another  we  gain  a  mass  of  knowledge 
which  it  is  very  desirable  to  express  in  a  concise  form. 
We  find  that  certain  compounds  resemble  one  another  very 
closely  in  their  conduct,  though  they  may  differ  markedly 
in  composition ;  we  learn  that  the  presence  of  certain  con- 
stituents in  a  compound  causes  it  to  act  in  a  particular 
way;  we  learn  that,  although  the  number  of  compounds  is 
unlimited,  the  number  of  classes  of  compounds  is  compara- 
tively small,  etc.  etc.  If  formulas  can  be  devised  which  will 
aid  us  in  expressing  intelligibly  the  results  of  investigation, 
they  must  be  of  value. 

From  the  earliest  periods  of  chemistry  formulas  of  this 
kind  have  been  used.  Without  going  back  to  the  begin- 
ning it  will  be  instructive  to  recall  the  general  character 
of  these  formulas  from  the  time  of  Lavoisier. 

This  chemist,  as  is  well  known,  paid  particular  attention 
to  the  phenomena  of  combustion.  He  regarded  all  com- 
pounds as  made  up  of  a  combustible  and  an  incombustible 
part.  In  his  formulas  he  separated  these  two  parts. 

When  the  electro- chemical  theory  held  sway  every  com- 
pound was  supposed  to  consist  of  an  electro-positive  and 
an  electro-negative  constituent.  Hence,  every  formula  con- 
sisted of  two  parts.  To  determine  the  formula  according 
to  this  theory,  a  compound  was  subjected  to  the  decom- 
posing influence  of  the  electric  current.  It  was  thus  sepa- 
rated into  two  parts,  and  these  parts  were  written  sepa- 
rately in  the  formula.  The  old  formulas  for  salts,  such  as 
NaO.SO3,  KO.NO5,  etc.,  which  we  see  even  at  the  present 
day,  particularly  in  works  on  mineralogy  and  analytical 
chemistry,  have  come  down  to  us  from  the  period  of  the 
electro  chemical  theory.  They  involve  more  speculation 
than  the  formulas  now  in  use,  and  that,  too,  of  a  kind  which 
has  been  shown  to  be  unfounded. 

The  next  idea  which  we  find  playing  an  important  part 


CONSTITUTION.  \  \  3 

in  the  construction  of  chemical  formulas  is  that  used  by 
Liebig.  This  chemist,  and  many  others  after  him,  effected 
the  decomposition  of  compounds  and  noticed  what  pro- 
ducts were  obtained.  In  writing  the  formula  of  the  original 
compound  they  indicated  in  it  the  presence  of  the  products 
they  had  obtained  from  it. 

After  this  came  the  idea  of  types,  which  was  developed 
by  Dumas  and  became  known  as  the  "theory  of  types." 
According  to  this  "  theory "  all  chemical  compounds  may 
be  referred  to  a  few  fundamental  compounds  or  types. 
All  compounds  belonging  to  the  same  type  are  constructed 
on  the  same  plan.  The  types  which  have  been  proposed 
up  to  the  present  time  are ; — 

I.  II.  III.  IV. 

HC1,  H20,  H3N,  H4C. 

Hydrochloric  acid.       Water.  Ammonia.  Marsh-gas. 

There  is  something  distinctive  in  each  of  these  com- 
pounds, and  the  traits  which  characterize  it  are  met  with 
in  the  compounds  belonging  to  the  same  type.  On  attempt- 
ing to  make  use  of  the  type  theory  for  the  purpose  of 
classifying  compounds  serious  difficulties  are  met  with, 
mainly  for  the  reason  that  many  compounds  belong,  not 
to  one  type,  but  to  several. 

The  introduction  of  the  idea  of  "mixed  types"  did  much 
to  overcome  these  difficulties,  but  still,  without  extension, 
the  idea  of  types  could  not  furnish  a  sufficient  basis  for 
formulas  which  should  express  the  principal  facts  known 
regarding  chemical  compounds.  The  "theory"  was,  how- 
ever, of  great  importance,  as  it  furnished  a  rational  method 
of  classifying  chemical  compounds,  and  directed  attention 
to  fundamental  differences  between  the  elements. 

An  examination  of  the  types  shows  that  the  elements 
chlorine,  oxygen,  nitrogen,  and  carbon  differ  from  one 
another  in  their  power  of  holding  hydrogen  in  combina- 
tion, and  leads  to  the  conclusion  that  the  reason  for  the 
existence  of  these  types  must  be  looked  for  in  the  nature 
of  the  elements  of  which  they  are  composed.  This  brings 
us  at  once  to  the  valency  hypothesis,  which  has  already 
been  discussed.  At  the  present  time  chemical  formulas 
are  based  upon  the  atomic  hypothesis,  the  hypothesis  of 
Avogadro,  and  the  valency  hypothesis.  As  there  is  a 
great  deal  of  misunderstanding  in  regard  to  these  formu- 


114    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

las,  as  by  some  they  are  overvalued,  and  by  others  under- 
valued, a  brief  statement  concerning  their  exact  significance 
is  desirable. 

It  cannot  be  denied  that  we  are  now  in  a  period  of  chem- 
istry which  may  fairly  be  called  one  of  formula  worship, 
More  value  is  sometimes  attached  to  a  formula  than  to 
that  which  it  is  intended  to  represent.  In  consequence  of 
this  it  has  happened  that  a  large  number  of  chemists  have 
regarded  the  determination  of  a  formula  for  a  compound 
as  the  great  object  to  be  accomplished,  and  they  have  for- 
gotten that  what  we  ought  to  know,  and  what  is  of  vastly 
greater  importance  for  the  science,  is  the  chemical  conduct 
of  the  compound.  If,  knowing  this,  we  can  represent  it 
by  means  of  a  formula,  not  only  are  we  justified  in  doing 
so,  but  the  formula  becomes  an  efficient  aid  in  dealing  with 
the  compound.  Formulas  have  been  proposed  for  nearly 
all  compounds  known.  Some  of  these,  indeed  many  of 
them,  are  valuable,  but  many  are  not.  Before  consider- 
ing the  means  at  our  command  for  deciding  whether  a 
formula  is  valuable  or  not,  a  few  words  in  regard  to  the 
general  methods  in  use  for  determining  formulas  will  be 
necessary. 

After  the  empirical  formula,  as  above  defined,  has  been 
determined,  the  next  thing  to  be  dojie  is  to  study  the  com- 
pound in  every  possible  way,  both  by  chemical  and 
physical  methods.  We  must  learn  exactly  how  it  con- 
ducts itself  under  all  circumstances  which  we  can  control. 
When  this  study  is  finished  we  shall  have  in  our  possession 
a  mass  of  facts.  We  shall  know  much  more  than  the  com- 
position, and  we  ought  to  be  able  to  express  much  more 
than  the  composition  by  our  formula.  It  is,  however,  by 
no  means  necessary  that  these  facts  should  be  expressed  in 
the  formula,  any  more  than  it  is  necessary  that  the  com- 
position should  be  expressed  in  a  formula.  Now,  how  can 
we  express  anything  in  regard  to  the  conduct  of  a  com- 
pound by  means  of  a  formula  ?  As  a  simple  example,  acetic 
acid  may  be  taken.  The  empirical  formula  is  easily  found 
to  be  C2H4O2.  We  find  that  metallic  elements  can  easily 
be  substituted  for  exactly  one- fourth  of  the  hydrogen  con- 
tained in  the  compound.  Thus,  with  potassium  hydroxide 
or  carbonate  we  get  C2H3O2.K.  Metalscannot  be  substi- 
tuted for  any  more  of  the  hydrogen,  so  that  we  are  justified 
in  concluding  that  one  of  the  four  atoms  of  hydrogen, 


CONSTITUTION.  115 

represented  in  the  empirical  formula,  differs  from  the  other 
three.  We  may,  hence,  write  the  formula  C2H3O2,H,  which 
expresses  the  difference  found  by  experiment. 

Further,  when  acetic  acid  is  treated  with  phosphorus 
trichloride,  it  is  converted  into  a  compound  of  the  formula 
C2H3O.C1  ;  that  is,  the  acid  loses  one  atom  of  hydrogen  and 
one  atom  of  oxygen,  and  in  place  of  them  takes  up  one 
atom  of  chlorine.  When  the  chlorine  compound  is  treated 
with  water,  acetic  acid  is  regenerated,  and  hydrochloric 
acid  is  formed  :  — 

C2H3O.C1  +  H20  =  C2H302.H  +  HC1. 

This  reaction  makes  it  appear  probable  that,  in  acetic  acid, 
one  of  the  oxygen  atoms  is  intimately  associated  with  a 
hydrogen  atom.  The  two  leave  the  acid  together,  and  enter 
it  together.  We  may  express  this  fact  by  the  formula 
C2H3O.OH.  The  chlorine  compound,  formed  by  treatment 
with  phosphorus  trichloride,  contains  no  hydrogen  replace- 
able by  metals,  so  that  it  appears  extremely  probable  that 
the  hydrogen,  which  is  represented  by  itself  in  the  formula 
C2H3O2.H,  is  the  same  as  that  represented  as  associated 
with  oxygen  in  the  formula  C2H3O.OH.  Similarly  it 
can  be  shown  that  the  second  oxygen  is  probably  asso- 
ciated with  carbon  in  the  same  way  as  it  is  in  carbon 
monoxide.  We,  hence,  write  CO.CH3.OH.  This  formula 
expresses  what  has  been  learned  by  a  study  of  the  reactions 
of  acetic  acid,  and  it  may,  hence,  be  called  a  reaction  for- 
mula. We  can  supplement  the  knowledge  gained  by  the 
reactions  above  referred  to  by  making  acetic  acid  from 
simpler  substances,  that  is,  by  the  process  of  synthesis. 
Thus,  for  example,  we  may  start  with  marsh-gas.  CH4,  and 
carbonyl  chloride,  COC12.  These  substances,  or  similar 
substances,  act  upon  each  other  as  represented  thus  :  — 

COC1  =  CH.COC1      HC1. 


If  the  product  CH3.COC1  is  found  to  be  identical  with  that 
above  mentioned  as  resulting  from  the  action  of  phosphorus 
trichloride  on  acetic  acid,  which  was  represented  by  the 
formula  C2H3O.C1,  then,  taking  into  consideration  the  facts 
above  mentioned,  we  are  led  to  the  formula  CH3.CO.OH 
for  acetic  acid,  This  formula,  as  far  as  it  is  based  upon 
the  synthesis  of  acetic  acid,  may  be  called  a  synthesis 
formula. 


116    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

The  reaction  formula  and  the  synthesis  formula  go  hand 
in  hand.  Both  express  facts  established,  and  are  of  value 
in  enabling  us  to  deal  with  the  facts.  Most  of  that  which 
is  of  value  in  the  formulas  in  common  use  is  independent 
of  the  hypothesis  of  valency.  The  formula  of  acetic  acid 
above  given  has  nothing  to  do  with  this  hypothesis,  and  if 
now  we  bring  in  the  hypothesis  to  aid  us,  it  is,  at  least, 
questionable  whether  we  gain  anything,  All  we  can  ac- 
complish by  means  of  it  is  to  account  for  the  employment 
of  the  hypothetical  affinities  or  bonds  of  the  elements.  We 
must  start  with  the  assumption  that  carbon  is  quadrivalent, 
oxygen  bivalent,  and  hydrogen  univalent,  and  then  bearing 
in  mind  the  reactions  above  described,  we  may  arrange  the 
formula  CHS  CO.OH  in  such  a  way  as  to  satisfy  the  bonds. 

H    O 

We  produce  thus  a  formula  like  this,  H — C — C — O — H, 

H 

which  is  a  fair  representative  of  what  are  known  as  consti- 
tutional or  structural  formulas. 

By  a  constitutional  or  structural  formula,  then,  we  mean 
one  which  expresses :  1st.  The  decompositions  of  the  com- 
pound ;  2d.  The  syntheses  of  the  compound  ;  and  3d.  The 
relations  existing  between  the  atoms  of  the  compound,  in 
terms  of  the  valency  hypothesis. 

It  must  be  distinctly  stated  that  we  cannot  use  the 
valency  hypothesis,  except  to  supplement  the  reaction  and 
synthesis  formulas.  We  are  not  justified  in  going  beyond 
the  facts  established.  Here  lies  the  danger  in  the  use  of 
structural  formulas.  Their  wholesale  use  to  express  some- 
thing about  which  we  know  absolutely  nothing  has  tended 
to  bring  them  into  disrepute,  but  this  fact  should  not  cause 
their  entire  rejection,  for  they  are  undoubtedly  of  the 
highest  value  when  rightly  used. 

In  the  following  parts  of  this  book  the  attempt  will  be 
made  to  show  upon  what  basis  the  structural  formulas  of 
the  principal  chemical  compounds  rest.  In  using  the  for- 
mulas the  student  should  accustom  himself  to  ask  in  every 
case  exactly  what  is  meant.  Above  all  things,  he  should 
not  be  satisfied  because  all  the  hypothetical  bonds  are 
satisfied. 


CONSTITUTION.  117 

Linkage  of  Atoms. — It  was  stated  above  that  "  most  of 
that  which  is  of  value  in  the  formulas  in  common  use  is 
independent  of  the  hypothesis  of  valency."  There  is,  how- 
ever, an  hypothesis  underlying  the  conception  of  valency, 
which  is  necessarily  involved  in  all  our  structural  formulas. 
This  is  the  hypothesis  of  the  linkage  of  atoms. 

It  is  plain  that,  considering  any  complex  compound,  say, 
acetic  acid,  C2H4O2,  there  are,  at  least,  two  views  possible 
in  regard  to  the  relations  of  the  constituents.  Either  these 
constituents,  or,  to  speak  in  terms  of  the  atomic  hypothesis, 
these  atoms,  are  all  united,  each  one  with  every  other  one, 
or  they  are  not  all  united  thus.  We  have  excellent  reasons 
for  believing  that  the  latter  is  the  correct  view ;  that  the 
atoms  are  united  or  linked  together  in  forms  which  may 
be  called  chains  with  branches.  We  are  forced  to  this 
view  by  an  overwhelming  array  of  facts,  prominent  among 
which  is  the  existence  of  the  so-called  homologous  series. 
The  relations  between  the  members  of  these  series  find  their 
simplest  explanation  in  the  assumption  that  the  carbon 
atoms  are  linked  together. 

We  have  the  series,  CH4,  C2H6,  C3H8,  C4H10,  etc.,  the 
members  of  which  very  closely  resemble  one  another. 
The  second  member,  C2H6,  can  be  made  from  the  first, 
by  introducing  chlorine  into  it,  the  product  CH3C1  being 
formed.  Now,  if,  under  proper  circumstances,  this  sub- 
stance is  treated  with  sodium,  the  chlorine  is  extracted, 
and  the  substance  C2H6  is  formed.  The  simplest  explana- 
tion of  these  reactions  is  this :  The  carbon  atom  in  methane, 
CH4,  holds  the  four  hydrogen  atoms,  and  can  do  nothing 
more.  Chlorine  cannot  be  added  to  this  compound,  but  it 
can  drive  out  hydrogen,  and  occupy  the  place  thus  made 
vacant.  Now,  if  the  chlorine  is  removed,  union  can  be 
effected  between  two  carbon  atoms,  and  according  to  this 
the  resulting  compound  must  be  represented  by  the  for- 
mula H3C — CH3,  which  indicates  that  the  carbon  atoms 
are  linked  together,  and  that  the  hydrogen  atoms  are  in 
combination  with  the  carbon  atoms.  It  may  safely  be  said 
that  all  the  facts  known  to  us  speak  in  favor  of  this  view. 

To  be  sure,  the  hypothesis  of  valency  is  also  involved  in 
this  explanation,  but  the  main  point  to  be  noticed  here  is, 
that  we  are  led  to  the  conclusion  that  the  atoms  are  linked 
together,  that  there  is  some  definite  arrangement  between 

them- 


118    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

Having  been  led  to  this  view,  the  problem  presents  itself 
in  the  case  of  every  compound  to  determine  how  the  atoms 
are  linked  together.  If  we  can  do  this,  we  can  determine 
the  constitution  of  the  compound,  and,  if  the  determina- 
tion is  properly  made,  the  result  is  almost  entirely  inde- 
pendent of  the  hypothesis  of  valency.  As  already  pointed 
out,  we  can  determine  these  relations  only  by  means  of 
experiment.  The  determination  is  never  absolute.  All 
that  we  can  say  is  that  the  compound  conducts  itself  as  if 
hydrogen  and  oxygen  were  united,  or  as  if  two  carbon 
atoms  were  linked  together,  and  we  then  make  use  of  a 
formula  to  recall  this  to  mind.  Such  relations,  established 
by  actual  experiment,  are  about  all  we  can  express  in  the 
formula,  and,  if  we  then  go  beyond  them,  and  distribute 
the  "  bonds"  so  that  all  are  satisfied,  we  are  dealing  with 
pure  hypothesis,  and  are  not  gaining  any  additional  in- 
sight into  the  nature  of  the  compound. 

If  the  question  is  asked,  What  is  the  meaning  of  the 
expression  "  linkage  of  atoms  ?"  the  only  answer  that  can 
be  properly  given  is,  that  it  is  simply  a  convenient  phrase 
to  indicate  the  condition  which  we  believe  to  exist  between 
the  smallest  parts  of  all  chemical  compounds.  As  to  its 
character  we  know  absolutely  nothing.  If  we  could  tell 
exactly  what  relation  the  smallest  particle  of  chlorine  bears 
to  that  of  hydrogen  in  hydrochloric  acid,  we  could  tell 
what  is  meant  by  "  linkage  of  atoms."  All  that  we  know 
is  that  the  chlorine  and  hydrogen  do  act  upon  each  other 
in  some  wonderful  way,  that  they  both  disappear  as  such, 
and  that  we  get  something  in  which  both  are  present. 
We  believe  that  the  act  of  union  takes  place  between  the 
atoms  of  the  elements,  and  we  represent  the  compound  by 
the  formula  HC1,  or  H.C1,  or  H— Cl.  It  is  not  at  all 
probable  that  a  firm  union  exists  between  the  two  parts 
in  such  a  way  as  to  render  the  parts  of  the  molecule  im- 
movable with  reference  to  each  other.  Much  more  likely 
is  it  that,  after  the  union,  the  atoms  perform  some  kind  of 
motion  with  reference  to  each  other,  according  to  the  laws 
of  atomic  motion  yet  to  be  discovered.  For  our  present 
purpose  it  is  sufficient  to  know  that,  in  whatever  manner 
the  union  takes  place,  the  chemical  activity  of  the  atoms 
is  differently  occupied  in  consequence. 

In  writing  a  structural  formula  we  do  not  commit  our- 
selves to  any  particular  view  in  regard  to  the  character  of 


CONSTITUTION.  119 

the  union  between  the  atoms,  nor,  generally,  in  regard  to 
the  relation  of  the  atoms  in  space.  All  that  we  can  attempt 
to  do  at  present  is  to  indicate  the  probable  connections  be- 
tween the  atoms.  Thus,  by  the  formula,  H3C — C — N,  we 
mean  that  the  reactions  and  methods  of  formation  of  the 
compound  represented  lead  to  the  conclusion  that  the  two 
carbon  atoms  are  closely  connected  or  linked  together,  while 
the  three  hydrogen  atoms  are  linked  to  one  of  the  carbon 
atoms  and  the  nitrogen  to  the  other.  The  reactions  of  the 
substance  and  the  methods  of  formation  lead  us  to  believe 
that  these  relations  exist  in  it.  If  we  go  further  and  rep- 
resent the  substance  by  the  formula  H3C — CEEN,  we  are 
then  simply  applying  the  hypothesis  of  valency,  and  we 
gain  little  if  anything.  We  know  practically  nothing  about 
the  relation  existing  between  the  carbon  and  nitrogen, 
though  the  above  formula  seems  to  indicate  that  the  re- 
lation is  a  firmer  one  than  that  expressed  by  the  simple 
line  as  C — N. 

The  formulas  in  use  for  the  various  classes  of  compounds 
known  to  us  will  now  be  considered,  more  particularly  with 
the  object  of  showing  the  connection  existing  between  the 
facts  and  the  formulas.  After  the  classes  have  been  dis- 
cussed, the  principal  compounds  of  each  class  will  be  taken 
up  and  treated  in  a  similar  manner.  In  this  section  the 
proofs  made  use  of  will  be  entirely  of  a  chemical  char- 
acter. In  a  subsequent  section  the  question  of  the  rela- 
tions between  physical  properties  and  constitution  will  be 
briefly  discussed. 


120    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 


CHAPTER    IX. 

CONSTITUTION   OF   CLASSES   OF   COMPOUNDS. 

CHEMICAL  compounds  can  be  most  conveniently  classi- 
fied according  to  their  chemical  properties.  No  system  of 
classification  that  has  been  proposed  up  to  the  present  can, 
in  any  sense,  be  called  perfect,  and  yet  the  system  in  com- 
mon use  is  convenient,  and  has  a  fair  foundation  of  facts. 

The  compound  which  has  the  formula  HC1,  hydrochloric 
acid,  and  the  compound  which  has  the  formula  KOH, 
potassium  hydroxide,  differ  very  markedly  from  each 
other.  The  former  has  a  sour  taste,  the  latter  has  the 
taste  of  lye,  or  an  alkaline  taste.  The  former  will  turn 
the  color  of  many  organic  substances,  while  the  latter  will 
undo  the  work  done  by  the  former,  restoring  the  original 
color.  In  whatever  way  we  may  consider  these  two  com- 
pounds, we  shall  find  that  they  have  opposite  or  comple- 
mentary properties.  They  are  both  chemically  active 
substances,  capable  of  producing  marked  changes  in  large 
numbers  of  other  compounds.  If  they  are  brought  together, 
they  neutralize  each  other ;  that  is  to  say,  they  destroy  each 
other's  characteristic  properties,  and  give  rise  to  the  forma- 
tion of  a  new  compound,  differing  entirely  from  the  two. 
The  two  compounds,  hydrochloric  acid,  HC1,  and  potassium 
hydroxide,  KOH,  are  representatives  of  two  great  classes 
of  compounds  known  as  adds  and  bases.  Many  of  the 
members  of  these  two  classes  possess  just  as  marked  prop- 
erties as  the  two  mentioned,  and  for  these  the  classification 
into  acids  and  bases  is  rational  and  simple.  But  there 
are,  further,  some  compounds  that  appear  to  possess  the 
characteristics  of  both  classes  to  a  certain  extent,  and  of 
neither  class  very  markedly.  They  act  like  acids  toward 
some  bases,  and  like  bases  toward  some  acids.  The  way 
they  act  depends  upon  the  character  of  the  substances  with 
which  they  are  brought  in  contact. 

Acids. — The  properties  which  characterize  acids  are  the 
following : — 


UNIVERSITY 

_  w  ^4 

Vg^CALIFOjaSX 

CLASSES  OF  COMPOUNDS:**  121 

1.  They  have  an  acid  or  sour  taste. 

2.  They  change  blue  litmus  red. 

3.  They  act  upon  metals,  hydrogen  being  evolved,  its 
place  being  taken  by  the  metals,  as,  for  instance :  — 

2HC1       +       Zn      =      ZnCl2       +       2H 

Hydrochloric  acid.  Zinc  chloride. 

H2SO4      +      Mn      =      MnSO4    +      2H. 

Sulphuric  acid.  Manganese  sulphate. 

4.  They  act  upon  metallic  hydroxides,  forming  neutral 
substances  and  water  as  follows : — 

HC1      +        KOH      =        KC1    +      H2O 

Hydrochloric  Potassium  Potassium 

acid.  hydroxide.  chloride. 

HNO3     +        NaOH     =      NaNO3  -f-       H2O 

Nitric  acid.         Sodium  hydroxide.       Sodium  nitrate. 

H2SO4     +       Ca(OH)2  =      CaSO4   +    2H2O. 

Sulphuric  acid.      Calcium  hydroxide.    Calcium  sulphate. 

Hydrogen  Acids. — All  acids  contain  hydrogen.  They 
may  consist  of  hydrogen  and  only  one  other  element,  or  of 
hydrogen  and  a  group  of  other  elements  of  greater  or  less 
complexity.  The  constitution  of  those  acids  which  consist 
of  hydrogen  and  only  one  other  element  is,  of  course,  very 
simple  and  readily  understood.  There  are  but  few  ex- 
amples of  this  kind,  among  which  are  hydrochloric  acid, 
HC1,  hydrobromic  acid,  HBr,  sulphydric  acid,  H2S,  etc. 
According  to  our  conceptions  of  the  nature  of  chemical 
constitution,  compounds  of  the  above  formulas  can  have 
only  one  constitution. 

It  is  a  noticeable  fact  that  acids  of  this  first  and  simplest 
class  never  contain  more  than  two  atoms  of  hydrogen  in 
the  molecule,  or  that  no  element  with  a  higher  hydrogen 
valency  than  two  forms  these  simple  acids. 

Hydroxyl  Acids. — By  far  the  greater  number  of  acids 
belong  to  the  second  class  mentioned.  They  consist  of 
hydrogen  and  a  group  of  greater  or  less  complexity,  as, 
for  instance,  H(NO3),  nitric  acid  ;  H(C1O3),  chloric  acid ; 
H2(SO4),  sulphuric  acid,  etc.  In  most  acids  of  this  kind, 
oxygen  is  one  of  the  constituents  of  the  group,  with  which 
the  hydrogen  is  combined. 

The  hydrogen  in  the&e  compounds  is  the  changeable 
constituent.  It  is  readily  given  up,  and  metals  and  groups 


122    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

are  taken  up  in  its  place.  The  first  question  that  suggests 
itself,  in  considering  the  constitution  of  acids,  is  this  :  In 
what  manner  is  the  hydrogen  in  them  held  in  combina- 
tion? It  is  believed  that  investigations,  thus  far  made, 
justify  the  answer  that  the  hydrogen  in  these  acids  is  gen- 
erally in  combination  with  oxygen,  and,  in  some  cases,  with 
that  element  which  is  so  similar  to  oxygen,  viz.,  sulphur. 
The  proofs  for  this  statement  cannot  always  be  given.  In 
the  case  of  many  acids  no  facts  are  known  that  prove 
that,  in  these,  the  hydrogen  is  combined  with  oxygen.  On 
the  other  hand,  there  are  so  many  acids  in  which  it  can  be 
satisfactorily  shown  that  the  hydrogen  is  thus  combined 
that  the  above  answer  seems  to  be  justified.  We  accord- 
ingly write  the  formulas  of  acids  in  such  a  way  as  to  indi- 
cate" the  fact  of  union  between  oxygen  and  hydrogen  thus  :  — 


(HO)NO2  2  22 

Nitric  acid.  Chloric  acid.  Sulphuric  acid. 

Or,  these  same  formulas  may  be  still  further  elaborated 
by  writing  them  as  follows  :  — 

H-O  ' 
H—  O—  NO2,    H—  O—  CIO,,  )SO2. 


Experimental  Evidence.  —  Under  certain  circumstances, 
an  atom  of  oxygen  and  an  atom  of  hydrogen  can  be  re- 
moved from  an  acid  containing  oxygen,  and  one  atom  of 
chlorine  introduced  in  the  place  occupied  by  the  displaced 
atoms.  Now,  the  simplest  conclusion  we  can  draw  is  that 
the  oxygen  and  hydrogen  were  present  in  the  compound  as 
a  univalent  group,  viz.,  as  hydroxyl  or  —  O  —  H. 
Thus, 

OH 

SQz\          yields  the  compounds 
XOH 

Cl  /Cl 

SO2(  and  S02( 

XOH  XC1 

Sulphuryl  oxychloride.  Sulphuryl  chloride. 


PO—  OH  yields  PO—  Cl. 

XOH  XC1 

Phosphoric  acid.  Phosphorus  oxychloride. 


CLASSES  OF  COMPOUNDS.  123 

C2H3O(OH)  yields  C2H3O—  Cl,  etc. 

Acetic  acid.  Acetyl  chloride. 

Another  reaction,  which  shows  plainly  that  in  these  acids 
hydrogen  is  intimately  associated  with  oxygen,  is  that  by 
which  the  group  NH2  is  introduced  into  them  in  the  place 
of  an  atom  of  oxygen  and  an  atom  of  hydrogen.  The 
fact  that  the  elements,  oxygen  and  hydrogen,  are  displaced 
together  indicates  a  connection  between  them  in  the  com- 
pound. 

We  have  the  following  instances  :  — 

C2H3O(OH)  yields  C2H3O(NH2) 

Acetic  acid.  Acetamide. 

C7H5O(OH)  yields  C7H5O(NH2) 

Benzoic  acid.  Benzamide. 

These,  with  other  general  reactions,  furnish  the  experi- 
mental evidence  in  favor  of  the  statements  above  made, 
that,  in  most  of  those  acids  which  contain  oxygen,  the 
characteristic  hydrogen  is  in  combination  with  oxygen  in 
the  form  of  hydroxyl  (OH).  In  some  cases,  as  already 
mentioned,  sulphur  takes  the  place  of  the  oxygen. 

Further  Experiments  necessary  in  most  Cases.  —  If  we 
accept  the  statement  that  hydroxyl  is  present  in  oxygen 
acids,  we  are  prepared  to  take  another  step.  This  hydroxyl 
may  be  in  combination  with  only  one  element  or  with  a 
group  of  elements.  If  it  is  in  combination  with  only  one 
element,  the  constitution  of  the  resulting  acid  is  easily 
understood.  For  instance,  in  the  compound  C1OH,  hypo- 
chlorous  acid,  only  one  method  of  combination  suggests 
itself,  viz.,  Cl  —  O  —  H.  There  are  but  few  examples  of 
this  kind. 

In  those  acids  in  which  the  hydroxyl  is  in  combination 
with  a  group  the  constitution  is  determined  when,  in  addi- 
tion to  showing  the  presence  of  hydroxyl,  the  special  con- 
stitution of  the  group  itself  is  determined. 

In  sulphuric  acid,  for  instance,  after  having  determined 
the  presence  of  two  hydroxyl  groups,  we  have  the  formula 

.OH 
SO2(         ;  but  this  formula  only  partially  expresses  the 


. 

constitution  of  the  acid.     It  remains  to  be  shown  in  what 

manner  the  atoms  are  combined  in  the  group  SO2,  and 


124    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

also,  with  what  atoms  the  hydroxyl  groups  are  combined. 
Under  the  assumption  that  in  sulphuric  acid  sulphur  is 
sexivalent,  and  oxygen  bivalent,  the  constitution  of  the 
acid  is  expressed  by  the  formula, 


. 

XO—  H 

Sulphur  Acids.  —  It  has  been  mentioned  that,  in  some 
acids,  sulphur  plays  the  part  which  oxygen  plays  in  the 
hydroxyl  acids.  In  these  we  have  the  univalent  group 
(SH).  The  grounds  for  assuming  the  presence  of  this 
group  in  a  compound  are  similar  to  those  which  lead  to 
the  assumption  that  the  group  (OH)  is  present.  The  two 
atoms  8  and  H  can  both  be  removed  from  the  compound 
and  be  replaced  by  one  univalent  atom,  as  chlorine;  and, 
farther,  there  is  a  general  tendency  on  the  part  of  these 
two  elements,  sulphur  and  hydrogen,  to  leave  the  com- 
pound together.  Examples  of  acids  of  this  kind  are 

SH 

SO2(  and  CN(SH). 

XOH 

Thiosulphuric  acid.  Sulphocyanic  acid. 

The  compounds  formed  when  the  sulphides  of  tin, 
arsenic,  and  antimony  are  dissolved  in  sulphides  of  the 
alkalies  are  in  all  probability  salts  of  sulphur  acids.  There 
are,  for  example,  the  salts, 

K,SnS3,  Na3AsS4,  Na3SbS4. 

These  are  salts  of  acids  of  the  formulas  :  — 

H2SnS3,  H3AsS4,  H3SbS4. 

These  salts  and  acids  are  clearly  analogous  to  the  better- 
known  oxygen  compounds  :  — 

K2Sn03,  Na3AsO4,  Na3SbO4, 

H2SnO3,  H3AsO4,  H3SbO4. 

And  probably  they  have  a  similar  constitution,  expressed 
by  the  formulas  :  — 

H—  Sx  H—  S\  H—  S\ 

)Sn=S,       H—  S—  As=S,      H-S—  Sb=S. 
H—  S/  H—  S/ 


CLASSES  OF  COMPOUNDS.  125 

Nitrogen  Acids. — Some  compounds  of  carbon  that  owe 
their  acid  properties  to  the  presence  in  them  of  hydrogen 
in  combination  with  nitrogen  have  long  been  known. 
Such,  for  example,  are  the  so-called  acid  imides,  as  succin- 
imide,  which,  as  will  be  shown  further  on,  has  the  con- 

CH2— COV  ,CO. 

stitution     |  )NH.  phthalimide,  C6H4(        )NH, 

CH.-CO7  XCOX 

CO 
benzoic  sulphinide,  C6H4^          /NH,  etc.     Then  there  are 

XSO/ 
some  other  similar  compounds  containing  the  group  NH, 

C6H5.CO 
which  also  have  acid  properties,  such  as,  /NH, 

C6H5.SO/ 
CH3.COX 

/NH,  etc.     In  general,  it  may  be  said  that  when 
CH3.CO/ 

acid  residues  are  substituted  for  two  of  the  hydrogen  atoms 
of  ammonia,  the  remaining  hydrogen  atom  has  acid  prop- 
erties, or  the  compound  thus  formed  is  an  acid. 

In  the  compound  azoimide,  or  hydrazoic  acid,  HN3,  we 
have  the  most  remarkable  example  of  the  acid  character 
of  the  group  NH.  The  evidence  all  goes  to  show  that 
the  structure  of  this  compound  is  that  represented  by  the 

N\ 
formula   ||    )NH. 

T*/ 

Finally,  in  one  form  of  cyanic  acid  it  is  probable 
that  this  imide  group  is  present,  as  represented  thus: 
O— C=N — H;  and  some  facts  seem  to  show  that  this 
group  is  also  present  in  hydrocyanic  acid,  as  shown  thus : 
C=N— H. 

Double  Halogen  Acids. — Many  salts  are  known  that 
are  derived  from  complex  acids  containing  the  halogens, 
fluorine,  chlorine,  bromine,  and  iodine.  Such,  for  example, 
are  the  salts  potassium  chlorplatinate,  K2PtCl6,  potassium 
fluosilicate,  K2SiF6,  sodium  chloraurate,  NaAuCl4,  potas- 
sium chlorstannate,  K2SnCl6,  etc,  These  salts  are  com- 
monly called  double  salts,  and  no  attempt  made  to  explain 
them.  A  careful  examination  has  shown  that  they  are 
analogous  to  the  oxygen  and  sulphur  salts  in  composition 


126    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

and  in  mode  of  formation,  and  it  seems  probable  that  they 
are  analogous  to  these  better-known  salts  also  in  structure. 
This  view  necessitates  the  assumption  that  a  double  halogen 
atom,  as  CJ2,  Br2,  etc.,  acts  as  a  bivalent  group,  playing 
the  same  part  as  oxygen  in  the  double  halogen  salts.  The 
only  objection  to  this  view  is  that  it  is  based  on  the  poly- 
valency  of  the  halogen  atoms.  This  objection  does  not 
appear  to  have  much  weight  in  view  of  the  fact  that  the 
polyvalency  of  the  halogens  toward  oxygen  is  now  gen- 
erally accepted.  The  analogy  between  the  oxygen,  the 
sulphur,  and  the  double  halogen  compounds  is  represented 
in  .the  table  given  below  :  — 

K2SnO3  K2SnS3             K2SnCl6 

K2Pt03  —                K2PtCl6 

KA102  KA1C1, 

K3AsO3  K3AsS3             K3AsCl6 

K2SiO3  K2SiF6 

As  regards  the  structure  of  the  double  halogen  atom  it 
seems  probable  that  it  is  made  up  thus,  —  Cl—  Cl  —  ,  the 
halogen  being  trivalent.  This  is  made  the  more  probable 
by  the  fact  that  there  are  some  salts  which  cannot  be 
explained  by  the  simple  assumption  represented  thus, 
—  Cl  —  Cl  —  ,  whereas  they  can  be  explained  by  the  other 
assumption.  The  structural  formula  of  a  single  double 
halogen  salt  will  show  how  the  others  are  to  be  represented 
if  the  hypothesis  above,  briefly  presented,  is  correct.  The 
analogy  between  the  oxygen,  sulphur,  and  double  halogen 
salts  is  shown  by  the  following  formulas  :— 


/0-K  /S—  K  /Cl,—  K 

0==Ba(  ,8=  Sn(  ,Cl2=Sn( 

X0—  K  XS—  K  XC12—  K 

Cl  C1=C1—  K 

or   the   last    may   be  written   thus,         /Sn^ 

CK       XC1=  Cl—  K 

It  should  be  added   that   several  of  the  double  halogen 

acids  are  known  in  the  free   state  as  chlorplatinic  acid, 

H2PtCl6,  chlorauric  acid,  HAuCl4,  etc. 

Classification  of  Acids.  —  It  will  be  seen  that  different 
acids  contain  different  numbers  of  hydroxyl  or  other  simi- 
lar groups  in  their  molecules.  An  acid  which  contains 


CLASSES  OF  COMPOUNDS.  127 

only  one  such  group  in  its  molecule  has,  of  course,  only 
one  acid  hydrogen  atom.  It  is  called  a  monobasic  acid. 
An  acid  which  contains  two  such  groups  in  its  molecule 
is  a  dibasic  acid.  We  have,  further,  tribasic,  tetrabasic 
acids,  etc. 

The  same  distinctions  are  possible  among  those  acids 
which  consist  of  hydrogen  combined  only  with  an  element, 
and  consequently  do  not  contain  hydroxyl;  but  as  of  these 
latter  acids  we  have  none  which  contains  more  than  two 
atoms  of  hydrogen  in  the  molecule,  so  we  have  among  them 
only  monobasic  and  dibasic  acids. 

Examples  :  — 

Monobasic  acids.  Dibasic  acids. 

HC1,  hydrochloric  acid.  .OH 

NO2(OH),  nitric  acid.  SO/        ,  sulphuric  acid. 

Cl(OH),  hypochlorous  acid.  XOH 

OH 

C,O2^         ,  oxalic  acid. 
XOH 

Tribasic  acids.  Tetrabasic  acids. 

/OH  P2O3(OH)4,  pyrophosphoric 

PO-OH  ,  phosphoric  acid.  acid- 

XOH 


AsO  —  OH  ,  arsenic  acid. 
XOH 

Bases.  —  Bases  have  properties  which  may,  in  general,  be 
said  to  be  the  opposite  of  those  of  acids.  They  generally  con- 
tain oxygen  and  hydrogen,  and  these  elements  are  combined 
as  hydroxyl,  as  may  be  shown  in  the  same  way  that  it  was 
shown  for  acids.  The  most  striking  characteristic  of  bases 
is  their  power  to  act  upon  acids,  forming  neutral  substances 
and  water,  as  is  expressed  in  the  following  equations  :  — 

KOH      +      HNO3      =      KNO3      +      H2O; 

Potassium  Nitric  Potassium 

hydroxide.  acid.  nitrate. 

Ca(OH)2    +      H2SO4      =      CaS04      +      2H2O. 

Calcium  Sulphuric  Calcium 

hydroxide.  acid.  sulphate. 


128    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

Almost  all  bases  consist  of  a  metal  combined  with 
hydroxyl.  A  few  appear  to  consist  of  a  group  of  atoms 
combined  with  hydroxyl. 

According  to  the  valency  of  the  metals  with  which  the 
hydroxyl  is  combined,  there  are  bases  with  one,  two,  three, 
etc.,  hydroxyl  groups  in  the  molecule.  Examples  of  these 
are  the  following : — 

K(OH),  potassium  hydroxide.  A1(OH  3,  aluminium  hydroxide. 

Na(OH),  sodium  hydroxide.  Cr(OH)3,  chromium  hydroxide. 

Ca(OH)2,  calcium  hydroxide.  Ti(OH)4,  titanium  hydroxide. 

Ba(OH)2,  barium  hydroxide.  Zr(OH)4,  zirconium  hydroxide. 

Differences  between  Acids  and  Bases. — The  difference 
between  acids  and  bases  is  dependent  upon  the  nature  of 
the  elements  or  groups  with  which  the  hydroxyl  is  com- 
bined. The  hydroxyl  compounds  of  those  elements  which 
have  a  markedly  metallic  character  are  bases.  The  hy- 
droxyl compounds  of  those  elements  which  have  a  markedly 
non-metallic  character  are  acids.  The  hydroxyl  compounds 
of  those  elements  which  are  neither  markedly  metallic  nor 
non-metallic  sometimes  act  as  acids  and  sometimes  as  bases. 
Thus  the  compound  SbO(OH),  antimonyl  hydroxide,  is  a 
weak  base  and  a  weak  acid,  exhibiting  one  class  of  proper- 
ties or  the  other,  according  to  the  nature  of  the  compound 
with  which  it  is  brought  in  contact. 

Further,  a  compound  consisting  of  a  metal  in  combina- 
tion with  hydrogen  and  oxygen,  and  possessing  basic  pro- 
perties, may  acquire  acid  properties  by  an  increase  in  the 
proportion  of  oxygen  in  it.  This  is  illustrated  by  alu- 
minium hydroxide,  A1(OH)3.  This  is  basic,  but  it  acquires 
acid  properties  by  loss  of  water.  It  is  thus  transformed 
into  the  compound  AIO(OH),  in  which  the  proportion  of 
oxygen  relatively  to  hydrogen  is  greater  than  in  the  basic 
hydroxide.  So,  too,  while  the  hydroxide  of  the  formula 
Fe(OH)2  is  basic,  the  compound  FeO.2(OH)2  is  acid.  A 
similar  fact  will  be  discussed  when  the  carbon  compounds 
are  presented.  It  will  then  be  seen  that  compounds  in 
which  the  group  H2C — O — H  is  present  are  basic,  but  that 
if  the  proportion  of  oxygen  in  the  group  is  increased  by 
the  introduction  of  oxygen  in  place  of  the  hydrogen  the 
substance  j  thus  formed  are  acid.  They  contain  the  group 
OC-O-H. 


CLASSES  OF  COMPOUNDS.  129 

Complex  Bases. — As  above  stated,  there  are  a  few  bases 
which  appear  to  consist  of  hydroxyl  combined  with  a  group 
of  atoms.  Such,  for  instance,  are 

BiO(OH)  UO2(OH)2  TiO(OH)2 

Bismuthyl  hydroxide.       Uranyi  hydroxide.       Titanyl  hydroxide. 

These  formulas  are  based  upon  the  assumption  that,  if 
one  molecule  of  a  base  has  the  power  to  neutralize  one 
molecule  of  a  monobasic  acid,  it  must  contain  one  hydroxyl 
group ;  if  it  can  neutralize  two  molecules  of  a  monobasic 
or  one  of  a  dibasic  acid,  it  must  contain  two  hydroxyls,  etc. 
Now,  bismuthyl  hydroxide,  or  the  quantity  of  substance 
represented  by  BiO2H,  has  the  power  to  neutralize  one 
molecule  of  a  monobasic  acid.  It  is,  as  we  say,  a  mon- 
acid  base,  and  contains  one  hydroxyl.  Hence  the  formula 
BiO(OH)  is  given  to  it.  We  have  no  means  of  deciding 
how  the  bismuth  and  oxygen  are  combined,  or  whether  the 
hydroxyl  is  in  combination  with  bismuth  or  with  oxygen. 
If,  however,  it  is  assumed  that  bismuth  is  trivalent  and  the 
formula  is  constructed  on  the  basis  of  the  valency  hypo- 
thesis we  have  O=Bi — O — H. 

Salts. — The  neutral  substances,  to  which  reference  has 
been  made,  formed  by  the  action  of  acids  upon  bases,  are 
called  salts.  Salts  may  be  considered  either  as  acids  in 
which  a  base  minus  hydroxyl  has  been  substituted  for  the 
hydrogen,  or  as  bases  in  which  an  acid  minus  hydroxyl 
has  been  substituted  for  the  hydrogen  .  As  the  base  resi- 
dues are  usually  simpler  than  those  of  the  acids,  the  former 
view  is  most  commonly  held,  although  the  two  views  are, 
of  course,  identical. 

It  is  a  simple  matter  to  deduce  the  constitution  of  a  salt 
from  that  of  the  acid  and  base  or  bases  from  which  it  is 
derived.  Usually  one  or  more  metallic  atoms  are  substi- 
tuted for  the  hydrogen  of  the  acid,  the  former,  as  is  be- 
lieved, being  held  in  combination  by  the  same  force  or 
forces  that  held  the  latter.  Thus  we  have 

/OH  .OK 

S02(  and  SO/ 

XOH  XOK 

Sulphuric  acid.  Potassium  sulphate. 

NO2— OH  and  NO2— ONa 

Nitric  acid.  Sodium  nitrate. 


130    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

Or,  a  bivalent  element  may  enter  into  an  acid,  in  which 
case  it  takes  the  place  of  two  hydrogen  atoms,  thus  :  — 

/OH  O. 

SO2(  and  SO2(       )Ca; 

XOH  ^O/ 

Sulphuric  acid.  Calcium  sulphate. 

N0a-0x 
NO—  OH  and  Ba. 


2 


Nitric  acid.  Barium  nitrate. 

Further  complications  are  introduced  when  trivalent  and 
quadrivalent  elements  enter  into  the  composition  of  salts. 
From  what  has  been  said,  however,  the  constitution  of  these 
salts  will  be  readily  understood. 

Complex  Salts.  —  Just  as  there  are  some  bases  which 
consist  of  hydroxyl  combined  with  groups  of  atoms,  so 
there  are  salts  which  may  be  considered  as  derived  from 
acids  by  the  substitution  of  groups  of  atoms  for  the  hydro- 
gen. Thus,  a  salt  obtained  from  the  acid  NO2  —  OH,  and  the 
base  UO2(OH)2,  probably  has  the  constitution  expressed 

NO-O 
by  the  formula  /UO2.    Here  the  group  U02,  which 


. 
is  bivalent,  is  substituted  for  the  hydrogen  of  the  acid. 

Anhydrides.  —  The  constituents  of  water  can  be  abstracted 
from  many  acids,  and  thus  a  new  class  of  compounds,  called 
anhydrides,  is  formed.  The  most  striking  characteristic  of 
these  compounds  is  their  power  to  form  acids  with  water, 
or  to  form  salts  by  direct  union  with  bases.  The  following 
are  examples  of  anhydrides:  Sulphuric  anhydride,  SO3; 
nitric  anhydride,  N2O5  ;  phosphoric  anhydride,  P2O5  ;  acetic 
anhydride  (C2H3O)2O,  etc. 

When  an  anhydride  is  formed  from  a  monobasic  acid, 
two  molecules  must  combine  to  furnish  the  hydrogen  for 
the  water.  After  the  abstraction  of  the  water,  the  two  acid 
residues  remain  united,  through  the  instrumentality  of  an 
atom  of  oxygen,  thus  :  — 

NO—  OH1  NO2V 

-    H20     =  )0; 

N02—  OH)  XO/ 

2  molecules  nitric  acid.  Nitric  anhydride. 


CLASSES  OF  COMPOUNDS. 


131 


C2H3— OH^) 
C2H3— OHJ 

2  molecules  acetic  acid. 


C2H30 

H20    =  )0. 

C2H3OX 

Acetic  anhydride. 


When  an  anhydride  is  formed  from  a  dibasic  acid  a 
molecule  of  water  may  be  given  off  from  a  molecule  of 
acid,  thus: — 

OH 

SO2(  —        H2O        =        S03; 

^011 

Sulphuric  acid.  Sulphuric  anhydride. 

,OH 


CO 


NOH 

Carbonic  acid. 


=       CO, 


Carbonic  anhydride. 


Or,  two  molecules  of  a  dibasic  acid  may  unite  and  give  off 
one  molecule  of  water,  forming  a  compound  which  is,  at 
the  same  time,  an  acid  and  an  anhydride,  thus : — 


SO, 


SO, 


OH 
.OH 


-    H20    = 


SO2 
SO2 


OH 

o  . 


XOH  f 

2  molecules  sulphuric  acid. 


:\OH 

Pyrosulphuric  acid. 


When  an  anhydride  is  formed  from  a  tribasic  acid  sev- 
eral possibilities  present  themselves.  1.  One  molecule  of 
the  acid  may  lose  one  molecule  of  water,  a  compound  being 
formed  which  is  anhydride  and  monobasic  acid,  thus : — 


H2O         =      PO2— OH. 

Metaphosphoric  acid. 


PO— OH    :rr: 

XOH 

Phosphoric  acid. 


2.  Two  molecules  of  the  acid  may  lose  one  molecule  of 
water,  a  compound  being  formed  which  is  a  tetrabasic  acid, 
and,  at  the  same  time,  an  anhydride  : — 


132    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 


PO— OH 
XOH 

/OH 
PO— OH 


-    H20    = 


/0H 
PO— OH 

>• 

PO-OH 
XOH 


XOH 

2  mol.  phosphoric  acid.  Pyrophosphoric  acid. 

3.  Two  molecules  of  the  acid  may  lose  three  molecules 
of  water,  a  complete  anhydride  being  formed : — 


:0. 


PO— OH 


/OH 
PO— OH 


OH  PO. 

3H20     = 


PO2 


XOH 

2  mol.  phosphoric  acid.  Phosphoric  anhydride. 

By  combining  a  number  of  molecules  of  the  acids  and 
abstracting  different  numbers  of  molecules  of  water  a  great 
variety  of  anhydrides  might  be  produced,  at  least  theoret- 
ically. Not  many  such  complicated  products  are  positively 
known,  however. 

From  tetrabasic  acids  and  acids  with  even  higher  ba- 
sicity, corresponding  anhydrides  may  be  derived.  With  an 
increase  in  the  basicity  of  the  acids  the  complexity  of  the 
resulting  anhydride  is,  of  course,  increased. 

Experimental  Evidence  of  the  Constitution  of  Anhydrides. 
—In  regard  to  the  correctness  of  the  formulas  given  for 
these  anhydrides,  it  can  only  be  said  that  they  are  the 
simplest  conceivable.  If  it  is  acknowledged  that  acetic 
anhydride,  (C2H3O)2O,  consists  of  two  acid  residues  united 
by  means  of  an  oxygen  atom,  then,  by  analogy,  it  would 
appear  that  the  other  anhydrides,  mentioned  above,  are 
constituted  as  represented  by  the  formulas.  But  can  we 
assume  any  other  formula  for  acetic  anhydride  ?  The  acid 
has  the  constitution  C2H3O(OH)  ;  the  anhydride  has  the 


CLASSES  OF  COMPOUNDS.  133 

empirical  formula  C4H6O3,  and  is  formed  by  the  simple 
abstraction  of  water  from  the  acid ;  it  is  known  that  the 
hydroxyl  group  has  the  power  to  separate  from  the  acid 
with  comparative  ease.  What,  then,  is  more  natural  than 
to  assume  that  the  water  which  is  given  off  from  the  acid 
is  formed  from  the  hydroxyl  groups,  and  that  the  groups 
C2H3O  remain  undecomposed  ?  But  this  would  give  us, 
besides  the  water,  two  groups,  C2H3O  and  an  oxygen  atom. 
These  are  all  combined  in  one  molecule,  and,  as  we  believe, 
in  such  a  way  that  the  oxygen  atom  joins  together  the  two 


-         \ 

groups  or  acid  residues,  giving  the  formula  /O. 

C2H30X 

When  an  anhydride  is  formed  by  the  abstraction  of  water 
from  one  molecule  of  an  acid  the  simplest  conclusion  is  that 
an  oxygen  atom  fills  the  place  before  occupied  by  two  hy- 
droxyl groups.  There  is  no  proof  of  this,  to  be  sure ;  but 
it  would  be  gratuitous  to  offer  any  other  explanation  of 
the  formation  of  these  anhydrides  at  present. 

Oxides. — Just  as  anhydrides  can  be  obtained  from  acids 
by  the  extraction  of  water,  so  the  oxides  may  be  regarded 
as  anhydrides  of  the  bases.  The  constitution  of  the  oxides 
is  simpler  than  that  of  the  anhydrides,  because  the  bases 
themselves  are  generally  simpler  than  the  acids. 

The  simplest  oxides  are  those  obtained  from  the  hy- 
droxides of  univalent  elements,  examples  of  which  fol- 
low : — 

KOH^  K^ 

H20  ^O; 

KOHJ  K/ 

2  molecules  potassium  Potassium 

hydroxide.  oxide. 

Na— OH')  Nav 

H20         =  )0; 

Na— OH  \  NaX 

Sodium  hydroxide.  Sodium  oxide. 

Of  oxides  obtained  from  the  hydroxides  of  bivalent 
elements  there  are,  among  others,  the  following : — 

/OH 

Ca(  H20        =        CaO; 

XOH 

Calcium  hydroxide.  Calcium  oxide. 


134    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

/OH 

Sr(  H2O  SrO. 


Strontium  hydroxide.  Strontium  oxide.  . 

Theoretically,  an  intermediate  anhydride  can  be  formed, 
from  either  of  the  two  preceding  oxides,  analogous  to  the 

OH 
S02( 

formation    of   pyrosulphuric    acid          /O    ,    from    two 

SO2( 

XOH 
molecules  of  sulphuric  acid,  thus  :  — 

Ca/OH]  /OH 

^k  pn  / 

\OH  I  HO      -  \0 

OH  f  "•"  f./°    ' 

Ca  \OH 


2  mol.  calcium  hydroxide.  Intermediate  anhydride. 

Some  compounds  of  this  kind  are  known,  derived  from 
copper,  iron,  etc. 

From  the  hydroxides  of  irivalent  elements  more  than 
one  oxide  can  be  formed.  If  one  molecule  of  the  hy- 
droxide loses  one  molecule  of  water,  a  substance  is  formed 
which  is  oxide  and  hydroxide  at  the  same  time.  Reference 
has  been  made  to  these  compounds  (see  ante,  p.  128)  under 
the  head  of  bases.  The  compound  A1O—  OH  may  be  re- 
garded as  derived  from  the  hydroxide  A1(OH)3,  by  the 
loss  of  one  molecule  of  water  from  one  molecule  of  the 
hydroxide.  It  is  both  oxide  and  hydroxide,  and,  as  has 
been  pointed  out,  it  has  acid  properties. 

The  most  common  method  of  formation  of  oxides  from 
hydroxides  of  trivalent  elements  consists  in  the  union  of 
two  molecules  of  the  hydroxide,  which  then  loses   three 
molecules  of  water,  thus  :  — 
,OH 


Al-OH 
XOH 

,OH 


AKX 
—      3H2O      —  ) 

A10X 


Al-OH 
XOH 

2  mol.  aluminium  hydroxide.  Aluminium  oxide. 


CLASSES  OF  COMPOUNDS.  135 

The  principle  of  the  formation  of  these  oxides  is  thus 
seen  to  be  the  same  as  that  of  the  formation  of  anhydrides. 
What  has  been  said  in  regard  to  the  constitution  of  the 
latter  holds  good  in  regard  to  the  constitution  of  the  former. 
The  view  stated  is  the  simplest  which  the  facts  permit. 

Analogy  between  Salts  and  Anhydrides  and  Oxides. — 
As  was  seen  above,  a  salt  is  either  an  acid  in  which  a  base 
residue  has  been  substituted  for  the  hydrogen,  or  a  base  in 
which  an  acid  residue  has  been  substituted  for  the  hydrogen. 
In  those  salts  which  are  derived  from  acids  containing  hy- 
droxyl  a  base  residue  and  an  acid  residue  are  united  by 
means  of  oxygen.  On  the  other  hand,  in  many  anhydrides 
two  acid  residues  are  united  by  means  of  oxygen,  while  in 
oxides  two  base  residues  are  united  by  means  of  oxygen. 


136    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 


CHAPTER  X. 

CONSTITUTION    OF    CLASSES    OF    COMPOUNDS    OF   CAKBON. 

WE  have  thus  briefly  considered  the  different  classes  of 
compounds,  and  have  seen  upon  what  foundations  our  ideas 
in  regard  to  the  general  constitution  of  these  classes  of 
compounds  rest.  Among  the  compounds  of  carbon  there 
are  many  representatives  of  each  of  the  classes  above  con- 
sidered, and  all  that  has  been  said  holds  good  for  these 
compounds;  but,  owing  to  some  peculiarities  of  carbon 
which  distinguish  it  from  the  other  elements,  certain  things 
hold  good  for  the  carbon  compounds  in  general  that  do  not 
hold  good  for  the  corresponding  compounds  of  other  ele- 
ments. In  the  following  paragraphs,  therefore,  the  general 
formulas  of  the  different  classes  of  carbon  compounds  will 
be  briefly  treated. 

Hydrocarbons. — Of  the  compounds  of  carbon,  those  which 
it  forms  with  hydrogen,  or  the  hydrocarbons,  are,  in  general, 
the  simplest,  and  of  the  hydrocarbons,  marsh-gas,  or  me- 
thane, CH4,  is  the  simplest  one.  With  our  present  ideas  in 
regard  to  constitution  there  can  be  but  one  formula  for 

H 


H— C— 


this  compound,  viz.:   H — C — H,  which  indicates  merely 

H 

that  a  quadrivalent  atom  of  carbon  is  saturated  by  four 
atoms  of  hydrogen.  This  is  the  most  rational  supposition 
that  can  be  made  with  reference  to  the  compound.  The 
formula  is  certainly  not  proved,  but  it  is  exceedingly  prob- 
able. As  marsh-gas  is  a  very  important  member  of  the 
group  of  carbon  compounds,  and  our  views  regarding  the 
constitution  of  other  hydrocarbons  are  based  very  largely 
upon  our  conception  of  marsh-gas,  it  will  be  well  to  inquire 
more  particularly  concerning  the  grounds  upon  which  the 
above  formula  is  based.  The  empirical  formula,  CH4,  is 


COMPOUNDS  OF  CARBON.  137 

first  established  by  means  of  analysis  and  the  determination 
of  the  specific  gravity  of  the  vapor  of  the  compound.  This 
formula  is  the  expression  of  a  fact  and  an  hypothesis.  The 
fact  expressed  is  that  methane  contains  75  per  cent,  carbon 
and  25  per  cent,  hydrogen.  The  hypothesis  is  that  the 
molecules  of  all  chemical  compounds,  in  the  form  of  gas 
or  vapor,  have  the  same  volume  as  a  molecule  of  hydrogen. 
This  hypothesis  tells  us  the  weights  of  the  atoms  contained 
in  the  molecule  of  methane  and  the  weight  of  the  molecule 
of  methane,  and,  hence,  further,  the  number  of  atoms  of 
carbon  and  hydrogen  contained  in  the  molecule.  Knowing 
the  above,  it  'remains  to  determine  in  what  manner  these 
atoms  are  united,  or,  which  is  the  same  thing,  to  determine 
how  the  atoms  are  linked  together.  A  study  of  the  various 
reactions  of  marsh-gas  shows  that  in  all  probability  each 
hydrogen  atom  exists  in  the  compound  independently  of 
the  others,  that  is,  not  connected  with  the  others.  Each  one 
can  be  removed  separately  and  again  introduced.  Further, 
there  are  no  grounds  whatever  for  believing  that  hydrogen 
ever  acts  in  any  other  way  than  as  an  univalent  element, 
and  the  relations  of  the  hydrocarbons  can  be  simply  repre- 
sented only  on  the  assumption  that  in  them  the  carbon  is 
quadrivalent.  Certainly  the  most  plausible  hypothesis  in 
regard  to  the  structure  of  marsh-gas  is  that  expressed  by 
H 

the  formula  H — C — H,  which  means  simply  that  in  the 


i 


molecule  of  the  compound  four  atoms  of  hydrogen  are  in 
combination  with  one  carbon  atom. 

A  question  which  naturally  suggests  itself  in  connection 
with  the  compound  CH4  is  this:  Are  all  the  hydrogen 
atoms  combined  in  the  same  way  in  the  molecule  ?  In  re- 
gard to  this  point,  it  can  only  be  said  that,  as  far  as  inves- 
tigations have  gone,  an  affirmative  answer  to  this  question 
seems  justified.  If  these  hydrogen  atoms  were  combined 
in  different  ways,  then,  by  replacing  different  ones  by  the 
same  element  or  group,  products  should  be  obtained  which 
are  not  identical.  No  such  results  have  been  reached,  al- 
though the  hydrogen  atoms  in  methane  have  been  replaced 
in  a  great  variety  of  ways.  It  is,  indeed,  in  these  facts  that 
the  commonly  accepted  formula  for  marsh-gas  finds  its 


138    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

H 
chief  support.     The  formula  H — C — H  is  the  only  one  by 

H 

which  each  of  the  four  hydrogen  atoms  can  be  represented 
as  bearing  the  same  relation  to  the  carbon  atom,  if  we 
accept  the  general  method  of  representation  for  which  the 
reasons  have  been  given  in  the  introductory  remarks  on 
constitution. 

Homologous  Series. — Starting  with  methane  there  is  a 
series  of  hydrocarbons  of  the  general  formula  CnH2a+2. 
These  resemble  one  another  in  many  respects,  and  differ 
from  one  another  in  their  formulas  in  a  very  simple  way. 
The  difference  between  the  formulas  of  any  two  contiguous 
members  of  this  series  is  CH2.  Such  a  series  is  called  an 
homologous  series.  A  number  of  similar  series  are  known. 
In  the  methane  series  there  are:  Methane,  CH4;  ethane, 
C2H6;  propane,  C3H8;  butane,  C4H]0,  etc. 

The  relation  between  the  members  of  this  series  is  ex- 
pressed in  formulas  which  represent  the  carbon  atoms 
combined  in  what  are  called  open  chains.  Thus,  as  has 
been  shown,  if  two  carbon  atoms  combine  in  the  simplest 
manner  possible,  viz.,  by  one  of  their  affinities  each,  a 
chain  is  formed  having  six  free  affinities,  as  follows : 

i    i 

— C — C — .  If  three  carbon  atoms  combine  in  the  same 
way,  a  chain  having  eight  free  affinities  is  formed,  thus : 
— C — C — C — .  In  the  same  way  four  carbon  atoms  com- 
bining give  a  chain  having  ten  affinities,  etc.,  etc.  By 
saturating  these  free  affinities  with  hydrogen  we  should 
get  compounds  of  the  formulas  C2H6,  C3H8,  C4H10,  etc., 
etc.,  which  are  the  formulas  of  the  hydrocarbons  above 
given. 

Experimental  Evidence. — The  results  of  certain  experi- 
ments furnish  strong  evidence  in  favor  of  the  views  in 
regard  to  the  nature  of  the  combination  in  the  methane 
series  of  hydrocarbons. 


COMPOUNDS  OF  CARBON.  139 

If  methane  is  treated  with  chlorine,  the  following  reac- 
tion takes  place  :  — 

H  H 


H—  C—  H        Cl—  Cl  =  H—  C—  C 


i 


+  Cl—  Cl  =  H—  C—  Cl  +  Cl—  H. 


Methane.  Chlormethane. 

If  the  product  is  treated  with  sodium,  the  chlorine  is  ex- 
tracted, and  a  compound  of  the  formula  C,H6  is  obtained 
according  to  the  following  equation  :  — 

H  H  H    H 

H—  C—  id  -f  Cli—  C-H  +  2Na  -=  H—  C—  C—  H  +  2NaCl. 

MM  II 

H  H  H    H 

4  molecules  chlormethane.  Ethane. 

With  ethane  similar  reactions   can  be  realized,  and  a 
product,  C3H8,  obtained,  thus  :  — 

H    H  H    H 

II  II 

1.  H-C—  C—  H  -f  Cl—  Cl  =  H—  C-C-C1  +  HC1. 

II  II 

H    H  H    H 

Ethane.  Chlorethane. 

H   H  H  H   H   H 

2.  H—  C—  C—  id  +  Cli—  C—  H  +  2Na  =  H—  C—  C—  C—  H+2NaCl. 


Chlorethane.          Clormethane.  Propane. 

It  is  plain  that,  by  continuing  these  reactions  with  the 
new  compounds  obtained,  we  have  it  in  our  power  to  build 
up  a  series  of  hydrocarbons  corresponding  to  the  series 
given  above.  If  the  combination  always  took  place  in 
the  manner  described,  we  should  have  simple  chains,  in 
which  all  the  carbon  atoms  except  those  at  the  ends  would 
have  two  free  affinities,  while  those  at  the  ends  would  have 
three.  The  hydrocarbons  themselves  would  be  represented 
respectively  by  the  formulas 

H,C.CH2.CH2.CH3;  H3C.CH2.CH2.CH2.CHS,  etc.,  etc. 
These   are  called    normal   hydrocarbons   to    distinguish 


140  '  PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

them  from  others  of  the  same  general  composition,  but  of 
different  constitution,  which  will  be  taken  up  in  a  later 
paragraph. 

Alcohols. — Running  parallel  to  the  series  of  hydrocar- 
bons which  have  just  been  considered  is  a  series  of  com- 
pounds which  are  regarded  as  derived  from  the  hydro- 
carbons by  the  substitution  of  the  univalent  group  OH,  or 
hydroxyl,  for  a  hydrogen  atom  in  each.  These  compounds 
are  to  organic  chemistry  what  the  hydroxides  of  the  metals 
are  to  inorganic  chemistry.  They  are  known  as  alcohols. 
The  simplest  of  these  is  derived  from  methane  and  has  the 
H 

formula  H— C— O— H. 


Evidence. — One  of  the  hydrogen  atoms  of  those  alcohols 
which  contain  but  one  oxygen  atom  differs  from  the  others. 
It  is  easily  replaceable  by  certain  groups,  known  as  acid 
groups,  which  will  be  treated  of  further  on.  It  is  also  re- 
placeable by  some  metals.  In  a  compound  of  the  formula 
CH4O  we  must  assume  that  one  hydrogen  atom  is  in  com- 
bination with  the  oxygen  atom,  while  the  other  three  are 
not,  in  order  to  account  for  its  characteristic  behavior. 
Again,  if  the  alcohol  is  treated  with  hydrochloric  acid,  the 
oxygen  atom  and  the  peculiar  hydrogen  atom  are  given 
off  together,  and  their  place  is  taken  by  a  single  atom  of 
chlorine.  This  shows  that  the  hydrogen  and  oxygen  were 
present  in  the  form  of  a  univalent  group,  or  as  hydroxyl, 
which  is  the  only  form  that  satisfies  these  conditions. 

H3C— OH  +  HO(N02)  =  H3C— O(NO2)  +  H2O. 

Methyl  alcohol.          Nitric  acid.  Methyl  nitrate.  Water. 

H3C— OH  +       HC1       '=       H3C— Cl       +  H2O. 

Methyl  alcohol.         Hydrochloric  Chlormethane.  Water, 

acid. 

H.C— OH  +        Na        =     H3C— ONa     +     H. 

Methyl  alcohol.  Sodium.  Sodium  methylate.        Hydrogen. 

Further,  the  hydroxyl  group  can  be  introduced  into  the 
hydrocarbons  and  the  alcohols  thus  obtained.  In  order  to 
obtain  the  alcohol  CH4O  we  may  start  with  chlormethane, 


COMPOUNDS  OF  CARBON^  141 

CH3C1.     If  this  is  treated  with  the  hydroxide  of  silver,  the 
following  reaction  is  realized  : — 

CH3C1     +     Ag(OH)     =    CH3.OH     +    AgCl. 

Chlormethane.  Methyl  alcohol. 

The  above  reactions  show  the  correctness  of  the  formula 
H 

H — C — O — H  for  the  first  member  of  the  series  of  alco- 


i 


hols.  Having  once  recognized  the  presence  of  hydroxyl 
in  this  alcohol,  we  should  naturally  expect  to  find  it  in  the 
other  alcohols.  It  is  found  in  them  all,  and  may  be  de- 
tected in  the  way  above  indicated. 

Classes  of  Alcohols. — It  has  been  found  that  there  are 
three  classes  of  alcohols,  called,  respectively,  primary,  secon- 
dary, and  tertiary.  These  differ  very  markedly  from  one 
another  in  their  properties. 

Primary  Alcohols. — The  differences  in  the  properties  of 
the  three  classes  of  alcohols  are  undoubtedly  due  to  differ- 
ences in  constitution.     In  all  primary  alcohols  the  group 
H 


CH2OH,  or  — C — 0 — H,  is  present.     This  was  seen  in  the 

H 

case  of  methyl  alcohol,  which  is   a   compound  of  this 

H 

group  with  hydrogen,  H — C — O — H.     In  ethyl  alcohol, 

H 

the  next  member  of  the  series,  this  group  is  also  present. 
This  follows  if  the  presence  of  hydroxyl  in  the  alcohols 

H   H 

is  accepted ;  for  in  the  compound  H — C — C — H  it  makes 

u 

no  difference  which  hydrogen  atom  is  replaced  by  hydroxyl, 

7* 


AX         n  J 
H    '    C 
OH 


142    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

the  resulting  compound  will,  in  every  case,  have  the  same 
constitution  and  will  necessarily  contain  the  group  CH2  OH. 
In  all  primary  alcohols  the  presence  of  the  group  CH2.OH 
can  be  proved  in  a  similar  way.  They  are  all  derived 
from  methyl  alcohol  by  the  replacement  of  a  hydrogen 
atom  by  hydrocarbon  residues  of  various  composition  and 
constitution. 

By  replacing  a  hydrogen  atom  by  methyl,  CH3,  ethyl 
alcohol,  CH3.CH2.OH,  is  obtained. 

By  replacing  a  hydrogen  atom  by  ethyl,  C2H5,  propyl 
alcohol,  C2H5.CH2.OH,  is  obtained. 

By  replacing  a  hydrogen  atom  by  propyl,  C3H8,  butyl 
alcohol,  C3H8.CH2.OH,  is  obtained,  etc. 

These  alcohols  may  also  be  represented  by  the  formulas, 
CH3  (-  C2H6  C  C3H8 

C  <{  }      ,    C  \  and     C  1  or,  in  general,  any 

OH  [OH 

fR 

I       TT 

primary  alcohol  by  the  formula  C  •{      ^   in  which  R  rep- 

[OH 

resents  any  univalent  group,  CH3,  C2H5,  etc.,  derived  from 
a  hydrocarbon. 

Secondary  Alcohols. — If  two  hydrogen  atoms  of  methyl 
alcohol  be  replaced  by  hydrocarbon  residues,  alcohols  are 
obtained  which  do  not  contain  the  group  CH2.OH,  as  is 
evident  from  the  following  examples  : — 

H  CH3  C2H5 

H— C— O— H,     H3C— C— O— H;     H3C— C-O-H. 

A  A  A 

Methyl  alcohol.  Isopropyl  alcohol.       Secondary  butyl  alcohol. 

These  substances  contain  the  group  CH.OH,  and  are 
representatives  of  secondary  alcohols. 

The  simplest  example  of  this  class  of  substances  is 
isopropyl  alcohol,  the  formula  of  which  is  given  above. 

Secondary  alcohols  may  also  be  represented  by  such 
formulas  as  the  following : — 


COMPOUNDS  OF  CARBON.  143 

r  CH,       r  CHS        (•  C2H5 

I   /^ITT  I   O  "FT  '   C1  FT 

C  <  TT  3>     C  <[  TT    5,     C  \  TT    5,  etc.,  or,  in  general, 
,  ±1  I   n  xi 


I 

t  OH       [OH        [OH 

R 


|     TDt 

by  the  formula  C  -j   TT   ,  in  which  R  and  R'  may  be  the 

[OH 

same  or  different  univalent  hydrocarbon  groups. 

Evidence  in  favor  of  the  General  Formula  of  Secondary 
Alcohols. — There  are  two  alcohols  of  the  formula  C3H8O. 
One  of  these  conducts  itself  like  the  primary  alcohols,  and 
hence  in  all  probability  contains  the  group  CH2.OH.  An 
alcohol  isomeric  with  the  primary  alcohol  cannot  contain 
the  group  CH2.OH,  but  must  contain  the  group  CH.OH, 
as  may  be  readily  shown.  Both  of  the  alcohols  are  de- 

H    H  H 

I      I      I 
rived  from  the  same  hydrocarbon,  H — C — C— C — H.     In 

H    H  H 

this  hydrocarbon  there  appear  to  be  only  two  kinds  of 
hydrogen  atoms,  viz.,  those  in  combination  with  the  central 
carbon  atom,  and  those  in  combination  with  the  terminal 
carbon  atoms.  If  any  one  of  the  latter  is  replaced  by 
hydroxyl,  primary  propyl  alcohol  containing  the  group 
CH2.OH  is  formed.  Whereas,  if  one  of  the  former  hy- 
drogen atoms  is  replaced  by  hydroxyl,  secondary  propyl 
alcohol  containing  the  group  CH.OH.  is  obtained.  Only 
these  two  cases  are  possible. 

But,  again,  this  secondary  alcohol  is  prepared  by  allow- 
ing nascent  hydrogen  to  act  upon  acetone.  It  will  be 
shown  that  acetone  must  be  represented  by  the  formula 
CH3 

CO.     Now,  in  its  conversion  into  secondary  propyl  alcohol 

CH3 

acetone  takes  up  two  atoms  of  hydrogen,  and  the  only 
place  where  these  hydrogen  atoms  can  find  entrance  into 
the  above  molecule,  if  the  carbon  is  quadrivalent,  is  in 
combination  with  the  central  carbon  atom.  If  the  oxygen 


144    PRINCIPLES  OF  THEORETICAL   CHEMISTRY. 

atom  is  linked  to  the  carbon  atom,  as  in  carbon  monoxide, 
a  condition  which  is  represented  by  the  formula  C  =  O, 
we  can  conceive  of  the  relation  between  the  carbon  and 
oxygen  being  changed  by  the  action  of  hydrogen,  so  that 
a  group  HC — O — H  may  be  formed.  This  is  what  is 
believed  to  take  place.  This  addition  of  hydrogen  changes 
the  relation  between  the  carbon  and  the  oxygen  and 
saturates  them : — 

C  H  CH 

II        +  I     - 

O  H  OH 

Similar  considerations,  in  connection  with  other  second- 
ary alcohols,  lead  to  similar  results,  and  hence  the  conclu- 
sion is  drawn  that  all  secondary  alcohols  contain  the  group 
CH.OH,  or  that  they  are  derived  from  methyl  alcohol  by 
the  replacement  of  two  hydrogen  atoms  by  univalent  hy- 
drocarbon groups. 

Tertiary  Alcohols. — If  three  hydrogen  atoms  of  methyl 
alcohol  are  replaced  by  hydrocarbon  residues,  alcohols  are 
obtained  which  contain  neither  the  group  CH2.OH  nor 
the  group  CH.OH,  as  is  shown  by  the  following  formu- 
las : — 

H  CH3  C2H5 

H— C— 0-H,    CH3— C— O— H,    CH3-C— O— H. 
H  CH3  CH3 

Methyl  alcohol.         Tertiary  butyl  alcohol.     Tertiary  amyl  alcohol. 

These  substances  contain  the  group  C.OH,  and  are  repre- 
sentatives of  tertiary  alcohols. 

The  simplest  example  of  this  class  of  alcohols  is  tertiary 
butyl  alcohol,  C4H10O,  the  constitution  of  which  is  indicated 
by  the  formula  given  above. 

The  tertiary  alcohols  may  also  be  represented  by  such 
'  CH3  f  CH,  (  C2H5 


formulas  as  C  ,     C  ,     C  1          >,  etc.,  or, 

'  ' 


OH  I  OH  [OH 

r  R 

~Rf 
in  general,  by  the  formula  C  «j   R/, ,  in  which  R,  R',  and 

OH 


COMPOUNDS  OF  CARBON.  145 

R"  may  be  the  same  or  different  equivalent  hydrocarbon 
residues. 

Experimental  Evidence. — The  evidence  for  the  formula 
CH3 

CH3 — C — OH,  for  the  simplest  tertiary  alcohol,  is  this : — 

CH3 

There  is  a  hydrocarbon,  the  formula  of  which  can  be 
CH3 

I 
shown  to  be  CH3 — C — CH3.     From  this  two  alcohols  are 

H 

derived,  one  of  which  conducts  itself  as  a  primary  alcohol, 
while  the  other  does  not.     The  former  must  have  the  for- 
CH3 

mula  CH3— C— CH2— OH.   The  only  alcohol  derived  from 

H 

the  hydrocarbon  which  is  not  a  primary  alcohol  must  have 
CH3 

the  formula  CH3 — C — CH3,  and  hence  contains  the  group 

O 

H 

— C — OH,  which  is  trivalent.  Further,  similar  considera- 
tions of  other  tertiary  alcohols  indicate  that  in  them  also 
the  group  C.OH  is  contained,  and,  consequently,  this  is 
looked  upon  as  the  characteristic  group  of  these  com- 
pounds. 

Very  strong  confirmatory  evidence  in  favor  of  the  com- 
monly accepted  views  as  to  the  structure  of  the  three  classes 
of  alcohols  is  furnished  by  the  study  of  the  changes  which 
they  undergo  when  treated  with  oxidizing  agents,  as  will 
be  shown  further  on. 


146    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

Mercaptans.  —  If,  in  place  of  hydroxyl  in  the  alcohols 
the  group  HS  is  introduced,  substances  called  mercaptans 
are  obtained.  These  are,  in  many  respects,  analogous  to 
alcohols,  though  in  their  reactions  they  differ  from  them 
somewhat.  Their  constitution  is  similar  to  that  of  the 
alcohols.  The  principal  method  for  their  formation  con- 
sists in  the  action  of  potassium  sulphydrate,  KSH,  on  the 
chlorides  of  hydrocarbon  residues,  the  reaction  taking  place 
as  follows  :  — 


+     K—  SH    =    R—  SH    +    KC1. 

Chloride.  Mercaptan. 

Theoretically,  a  mercaptan  can  be  prepared  corresponding 
to  every  alcohol.  Thus  we  might  have  primary,  secondary, 
and  tertiary  mercaptans,  corresponding  to  all  the  known 
primary,  secondary,  and  tertiary  alcohols.  Only  such  mer- 
captans as  correspond  to  the  primary  alcohols  have  been 
prepared  up  to  the  present. 

Acids.  —  What  has  already  been  said  concerning  acids 
in  general  is  true  of  the  acids  of  carbon.  They  contain 
hydroxyl,  and  possess  the  general  properties  of  acids.  In 
general,  they  are  weaker  than  other  acids,  though  they 
differ  in  strength  between  comparatively  wide  limits.  There 
are  several  series  of  acids  of  carbon,  corresponding  to 
the  series  of  hydrocarbons  and  alcohols.  The  simplest 
carbon  acid  is  derived  from  methane,  and  has  the  formula 
H 

»     It  differs  from  the  simplest  alcohol  in 
O=C—  O—  H 

containing  an  atom  of  oxygen  in  the  place  of  two  atoms  of 
hydrogen.  This  is  clearly  shown  by  writing  the  formulas 
of  the  alcohol  and  of  the  acid  side  by  side  in  this  way  :  — 

H  H 

H2=C—  0-H  o-C-O-H. 

Alcohol.  Acid. 

Just  as  the  alcohol  consists  of  hydrogen  combined  with  the 
group  CH2.OH,  so  the  acid  consists  of  hydrogen  combined 
with  the  group  CO.  OH.  This  is  the  characteristic  group 
of  the  acids  of  carbon. 


COMPOUNDS  OF  CARBON.  147 

Experimental  Evidence. — In  the  first  place,  the  presence 
of  hydroxyl  is  proved  as  in  the  case  of  ordinary  acids.  As- 
suming the  presence  of  hydroxyl  for  reasons  already  given, 
the  formula  of  the  acid,  H2CO2,  becomes  HCO— OH.  Fur- 
ther, the  other  hydrogen  atom  contained  in  the  acid  does 
not  conduct  itself  as  if  it  were  in  combination  with  oxygen, 
but  rather  like  hydrogen  atoms  which  are  in  combination 
with  carbon  directly.  No  changes  which  the  acid  under- 
goes indicate  any  connection  between  this  hydrogen  and 
oxygen,  and  we  may  therefore  conclude  that  they  are  not 
present  as  hydroxyl.  But  if  they  are  not  present  as 
hydroxyl,  they  must  be  united  directly  with  the  carbon 

*  H 

atom,  and  the  formula  is  .    Now,  by  certain 

0=C— 0-H 

reactions,  it  is  possible  to  replace  that  hydrogen  atom  in 
the  acid  which  is  in  direct  combination  with  the  carbon 
by  groups  such  as  CH3,  C2H5,  etc.  The  compounds  thus 
obtained  must  contain  the  group  CO.  OH.  They  possess 
all  the  properties  of  acids. 

Methods  for  the  Formation  of  the  Acids  of  Carbon. — The 
methods  of  preparation  of  the  acids  of  carbon  throw  light 
upon  their  constitution.  Some  of  these  methods  are  here 
briefly  described. 

1.  The  simplest  acid,  above  referred  to,  viz.,  formic  acid, 
H2CO2,  is  obtained  by  bringing  carbon  monoxide,  CO, 
together  with  potassium  hydroxide,  KOH.  The  two  sub- 
stances combine  directly,  yielding  the  potassium  salt  of  the 
above  acid,  thus : — 

CO     +     KOH  HC02K. 

From  this  experiment  we  conclude  that,  in  the  salt,  one  of 
the  oxygen  atoms  is  in  direct  combination  with  carbon,  as 
it  was  in  carbon  monoxide,  while  the  other  oxygen  atom 
serves  the  purpose  of  linking  the  carbon  atom  to  potassium. 
Hence  the  group 

COOK  or  O-C— O— K 

I 
is  present  in  the  salt,  and  the  group 

O— C-O— H  in  the  acid. 

This  proof  is  unsatisfactory,  for  a  similar  argument  might 


148     PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

be  used  to  show  that  the  hydrogen  in  the  salt  HCO2K  is 
in  direct  combination  with  oxygen, 

2.  When  certain   hydrocarbons  are  allowed  to  act  upon 
carbonyl  chloride,  COCJ2,  one  of  the  chlorine  atoms  is  re- 
placed by  a  residue,  as  may  be  illustrated  thus:  — 
(I.)  CH4     +     COC12    =     CH3.COC1    +     HC1. 

When    the    product    is   treated  with  water  the  second 
chlorine  atom  is  replaced  by  OH,  as  follows  :  — 

(II.)  CH3.COC1  +  HHO  ==  CH3.CO.OH  +  HC1. 

Now  carbonyl  chloride  is  obtained  by  the  direct  addition 
of  chlorine,  CL,  to  carbon  monoxide,  CO  ;  and  hence  has 

Cl 
the  constitution  CO'      .     The  simplest  interpretation  of 

.    Ci 
reaction  (I.)  above  is  that  the  residue  CH3  takes  the  place 

occupied  by  one  of  the  chlorine  atoms,  which  would  give 


/ 

CO'         .     Lastly,  the  simplest  interpretation   of  reac- 

01 

tion  (II.)  is  that  the  hydroxyl  group  enters  in  the  place  of 
the  second  chlorine  atom,  which  gives  us  the  constitution 

CH3 
of  the  product  CO'         .     This  product  is  acetic  acid,  a 


homologue  of  the  simplest  acid  of  carbon.     It  contains  the 
group  CO.OH. 

CN 
3.  The  compound  cyanogen,    |     ,  is  converted  into  an 

CN 

acid  by  the  action  of  water.  This  acid  has  the  formula 
C.2H2O4.  It  is  a  dibasic  acid,  and  hence  contains  two  hy- 
droxyl groups,  which  would  lead  to  the  formula  C2O2(OH)2. 
As  both  the  hydroxyl  groups  conduct  themselves  in  exactly 
the  same  way,  it  is  concluded  that  they  are  combined  in 
exactly  the  same  way.  The  only  formula  that  satisfies 
OH 


these  conditions  is   |     .     In  this  compound  there  are  two 
OH 


COMPOUNDS  OF  CARBON.  149 

roups  CO.OH,  and,  as  we  have  seen,  it  is  a  dibasic  acid. 

"here  are  a  great  many  compounds  containing  the  groups 
CN  acting  as  a  univalent  group.  By  treating  these  with 
solutions  of  metallic  hydroxides  the  nitrogen  is  given  off 
in  the  form  of  ammonia,  NH3,  and  in  its  place  two  atoms 
of  oxygen  and  one  atom  of  a  univalent  element  are  taken 
up.  The  group  with  which  the  CN  is  in  combination 
remains  unchanged.  Hence,  in  accordance  with  the  above 
experiment,  it  is  believed  that  this  reaction  consists  in  a 
conversion  of  the  group  CN  into  COOH  or  COOM,  in 
which  M  represents  one  atom  of  a  univalent  metal.  The 
constitution  of  this  group  is,  of  course,  expressed  as  above 

O—  P—  OH 

by  the  formula  ,  .     All   the  substances   thus 

prepared,  and  containing  this  group,  are  derivatives  of  the 
acids  ;  they  are  salts. 

The  methods  of  formation  and  the  reactions  of  the  or- 
ganic acids  lead  then  to  the  conclusion  that  they  are  made 

/B  fB 

up  as  indicated  in  the  general  formula  CO(^         or  C  •<  O 

XOH         (OH, 
according  to  which  they  are  derivatives  of  carbonic  acid, 

OH 
CO^         ,  being  derived  from  it  by  the  introduction  of  a 

XOH 

residue  R  in  place  of  one  hydroxyl  of  the  acid.  The  rela- 
tions between  a  primary  alcohol,  a  cyanide,  and  an  acid 
are  shown  by  the  following  formulas  :  — 

R  fR 

,  C^O    . 

N  OH 


Aldehydes.  —  Aldehydes  are  products  formed  by  the  par- 
tial oxidation  of  primary  alcohols,  the  group  CH2OH 
being  converted  into  COH.  This  group  is  not  identical 

with  the  group  —  C  —  O—  H  of  tertiary  alcohols,  but  has 

the  constitution  expressed  by  the  formula  O=C  —  H.     It 
is  univalent,  just  as  the  group  CH2OH,  from  which  it  is 


150    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

derived,  is  univalent ;  whereas,  the  tertiary  alcohol  group, 
COH,  is  trivalent.  The  aldehydes  are  intermediate  pro- 
ducts between  primary  alcohols  and  the  acids  which  these 
yield.  It  was  shown  that  the  acids  are  formed  from  these 
alcohols  by  the  extraction  of  hydrogen  and  addition  of 
oxygen.  If  hydrogen  is  abstracted,  and  no  oxygen  added, 
the  product  is  an  aldehyde,  thus : — 

R— CH2OH,        R— COH,        R— COOH. 

Primary  alcohol.  Aldehyde.  Acid. 

Experimental  Evidence. — The  proofs  of  the  general  con- 
stitution of  aldehydes  are  similar  to  those  given  for  the 
acids.  Take,  for  instance,  the  simplest  aldehyde.  This 
has  the  formula  H2CO.  The  reactions  of  the  substance 
show  that  hydroxyl  is  not  present.  If  treated  with  the 
chlorides  of  phosphorus,  the  oxygen  of  an  aldehyde  is  re- 
placed by  two  chlorine  atoms.  This  shows  that  the  oxygen 
is  held  in  combination  by  the  carbon  atom  in  quite  a 
different  way  from  that  which  is  characteristic  of  hy- 
droxyl, and,  consequently,  it  cannot  be  in  combination 
with  hydrogen,  forming  hydroxyl.  This  leads  to  the  for- 

H  H 

mula  or  for  the  above  compound. 

O— C— H         0=C— H 
It  consists  of  a  hydrogen  atom  combined  with  the  group 

H 
[        .     Other  aldehydes  are  derived  from  this  simplest 

one  by  substituting  a  residue  of  greater  or  less  complexity 
for  one  of  the  hydrogen  atoms.  Thus,  the  group  CH3  or 
C2H5  may  be  introduced,  and  the  compounds  CH3 — COH 
and  C2H5 — COH  formed,  both  of  which  are  aldehydes. 

The  methods  for  the  preparation  of  aldehydes  also  fur- 
nish evidence  in  favor  of  the  constitution  above  ascribed 
to  them.  Some  of  these  are  the  following : — 

1.  It  has  already  been  seen  that  when  an  acid  is  treated 
with  the  chlorides  of  phosphorus  an  atom  of  chlorine  is 
substituted  for  its  hydroxyl.  Each  such  chloride,  as  was 

Cl 
shown,  contains  the  group     |        .     If  hydrogen  could  be 

substituted  for  the  chlorine  atom  in  this  group,  the  charac- 


COMPOUNDS  OF  CARBON.  151 

H 

teristic  aldehyde  group       |          would  plainly  be  formed. 

Such  a  substitution  has  been  effected  in  the  case  of  some 
of  the  chlorides,  and  the  resulting  compounds  have  been 
found  to  be  the  expected  aldehydes. 

2.  When  a  salt  of  any  acid  of  carbon  is  mixed  with  a 
salt  of  the  simplest  acid  of  carbon  (formic  acid),  of  the 
formula  H.CO.OH,  and  the  mixture  distilled,  an  aldehyde 
is  formed,  together  with  a  carbonate.  The  carbonates 
are  derived  from  a  dibasic  acid,  and  have  the  formula 

OM 
C0(        .     It  is  rational  to  suppose  that  the  groups  OM 

XOM 

have  passed  directly  from  the  compounds  in  which  they 
were  originally  contained  to  the  carbonate,  and  that  the 
group  CO  has  also  been  derived  directly  from  one  of 
the  original  acids.  If  these  suppositions  are  correct,  then 
we  are  led  to  the  conclusion  that  the  aldehyde  resulting 

H 
from  the  described  reaction  contains  the  group  |         .     For, 

C— O 

let  R — CO.OM  represent  the  formula  of  any  salt  of  a 
carbon  acid,  and  H.CO.OM  a  salt  of  formic  acid.  On 
bringing  these  two  compounds  together  and  heating  them, 
either  one  of  two  things  can  take  place  if  the  above  sup- 
positions be  correct.  The  groups  forming  the  carbonate 
may  be  split  off  thus  : — 


R— CO— OM 
OM 


H— CO- 

or,  thus : 

R_CO— 


OM 


H— 


CO— OM 


The  remaining  groups,  uniting  in  the  simplest  way,  will 

H 
give,  in  either  case,  a  compound, 

R— C— O 
The  relation  between  a  primary  alcohol  and  the  corre- 


152    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

spending  aldehyde  and  acid  is  shown  by  the  following 

formulas : — 

-p 

TT  \R 

CNH  COH 


OH 

Alcohol.  Aldehyde.  Acid. 

Acetones. — Acetones  are  products  of  the  partial  oxida- 
tion of  secondary  alcohols,  the  group  — C — OH  being  con- 

H 
verted  into  CO.     The  aldehydes,  too,  contain  the  group 

;  but  it  is  further  characteristic  of  aldehydes  that  one 
of  the  affinities  of  this  group  is  saturated  with  hydrogen, 
giving  the  complete  group  CO.  On  the  other  hand,  it  is 

H 

characteristic  of  acetones  that  both  of  the  affinities  of  the 

I 
group  CO  are  saturated  with  hydrocarbon  residues.     Thus 

CH3 

the  simplest  acetone  has  the  formula  CO  ,  both  of  the 

OH, 

affinities  of  the  characteristic  group  being  saturated  with 
residues  of  the  hydrocarbon  methane,  CH4. 

Experimental  Evidence. — As  just  stated,  the  simplest  ace- 
tone has  the  formula  C3H6O.  If  a  chloride  of  phosphorus 
is  allowed  to  act  upon  this  compound,  the  result  is  similar 
to  that  obtained  in  the  same  experiment  with  aldehydes, 
viz.,  the  atom  of  oxygen  is  abstracted,  and  two  chlorine 
atoms  take  its  place.  This  shows  that  the  oxygen  was  not 


COMPO  UNDS  OF  CARBON.  ]  53 

present  as  hydroxyl,  but  was  combined  with  the  carbon 
atom,  as  in  carbon  monoxide,  forming  the  group  CO. 

Again,  if  nascent  hydrogen  is  allowed  to  act  upon  this 

acetone,  secondary  propyl  alcohol  is  the  product,  and  the 

CH, 


alcohol  has  the  formula  C  (         .     From  this  we  may  con- 
I  XH 
CH3 

elude  that  in  acetone,  as  well  as  in  secondary  propyl  alco- 
hol, the  two  groups  CH3  are  present;  and  we  are  thus  led 
CH3 

to  the  formula  CO  for  the  simplest  acetone.     It  plainly 


consists  of  two  hydrocarbon  residues  combined  by  means 
of  the  bivalent  group  CO. 

The  following  methods  of  preparation  serve  as  evidence 
of  the  accepted  constitution  of  acetones : — 

1.  Just  as  aldehydes  are  obtained  from  acid  chlorides  by 
substituting  hydrogen  for  the  chlorine,  so  acetones  are  ob- 
tained from  the  same  chlorides  by  substituting  hydrocarbon 
residues  for  the  chlorine.  By  treating  acetyl  chloride, 
C2H3O.C1,  with  zinc  methyl,  Zn(CH3)2,  ordinary  acetone, 
CO(CH3)2,  is  produced  together  with  zinc  chloride,  ZnCl2. 

O 

The  formula  of  acetyl  chloride  is  known  to  be  CH3 — C — Cl. 
The  simplest  interpretation  of  the  above  reaction  is  that  a 
methyl  group  of  zinc  methyl  takes  the  place  of  a  chlorine 
atom  in  acetyl  chloride,  thus : — 

,0  X0 


CH3-C- 


Cl          I  XCH3      CH3— C— CHS 

-f  Zn(  4  ZnCla. 

Cl |  XCH3      CH3— C— CH3 


154    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

And  this  leads  clearly  to  the  formula  above  given  for 
acetone. 

2.  When  the  salts  of  many  of  the  acids  of  carbon  are 
subjected  to  dry  distillation  acetones  are  formed,  together 
with  a  carbonate  or  carbonates.  This  reaction  is  analogous 
to  the  reaction  for  the  preparation  of  aldehydes,  by  the 
distillation  of  a  mixture  of  the  salt  of  some  carbon  acid 
and  a  salt  of  formic  acid.  What  has  been  said  in  regard 

to  the  latter  reaction,  showing  that  the  group  C— O  must 

be  present  in  aldehydes,  holds  good  in  regard  to  the  reaction 
under  consideration,  and  shows  just  as  conclusively  that  the 

group  C — O  must  be  present  in  acetones.     Let  R.CO.OM 

represent  a  salt  of  an  acid  of  carbon.     Its  decomposition 
by  heat  may  be  represented  as  follows : — 


R.   CO.OM 


R.   CO.|OM 

The  residues  uniting,  a  compound,  R — CO — R,  is  formed? 
which  has  the  general  formula  of  an  acetone.  Or  let 
R.COOM  represent  the  salt  of  one  carbon  acid  and 
R'.COOM  the  salt  of  another  carbon  acid,  in  which  R 
and  R'  are  both  hydrocarbon  residues.  The  decomposi- 
tion which  takes  place  when  a  mixture  of  these  two  salts 
is  heated  is  represented  as  follows : — 


R. 


COOM 


R'.   COOM 

This  gives  a  compound  of  the  formula  R — CO — R'. 

It  will  be  seen  that  one  of  the  first  conditions  for  the 
production  of  an  acetone  by  means  of  this  reaction  is  that 
neither  of  the  salts  employed  should  be  a  formate,  H.COOM, 
as  the  use  of  the  latter  would  lead  to  the  formation  of  an 
aldehyde. 

The  facts  that  when  a  primary  alcohol  is  oxidized  it  is 
first  converted  into  an  aldehde  and  then  into  an  acid 


COMPOUNDS  OF  CARBON.  155 

that  a  secondary  alcohol  first  yields  acetone  and  then 
breaks  down;  and  that  a  tertiary  alcohol  yields  neither 
acetone  nor  aldehyde,  but  breaks  down — these  facts  are 
in  perfect  accordance  with  the  views  held  regarding  the 
constitution  of  the  compounds,  and  find  a  ready  inter- 
pretation by  their  aid.  To  make  this  clear  a  few  words 
of  explanation  are  necessary.  It  has  been  found  that, 

fE,  . 

when  a  hydrocarbon  of  the  general  formula  C  <  TW,  L 

[H 

treated  with  an  oxidizing  agent,  the  three  residues  at  first 
resist  the  action  and  the  hydrogen  atom  is  changed  to 
hydroxyl.  From  a  study  of  a  large  number  of  facts  it 
appears  clear  that,  whenever  a  compound  containing  hy- 
drogen in  combination  with  carbon  is  oxidized,  the  first 
change  brought  about  is  the  change  of  hydrogen  to 
hydroxyl.  Assuming  this  to  be  the  case,  the  first  change 
of  a  primary  alcohol  would  be  represented  thus : — 

fR  fE 

^   _i_   n  -     r« J  -^ 

H   +  1  OH. 

OH  L  OH 

But  the  compound  thus  formed  evidently  represents  an 
unstable  condition.  Compounds  containing  two  hy- 
droxyls  in  combination  with  one  carbon  give  up  water 
readily,  as  is  seen  in  the  case  of  ordinary  carbonic  acid, 

OH 
CO^        ,  which,  as  is  well  known,  loses  water  spontane- 

XOH 

ously,  and  is  thus  converted  into  carbon  dioxide,  CO2.  If 
this  change  should  take  place  in  the  compound  represented 

(R 
above,  the  result  would  be  an  aldehyde,  C  •<  H .      Now, 

(.0 

by  further  oxidation,  the  remaining  hydrogen  would  be 
changed  to  hydroxyl,  and  the  product  would  be  an  acid, 

CJOH. 

io 

As  regards  the  oxidation  of  secondary  alcohols  to  ace- 
tones a  similar  explanation  holds  good.  The  secondary 


156     PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 


R 
R' 

H 
OH 


TW 

alcohol    C    •(  TT     would  first  be  converted  into  the  com- 
±1 


T>/ 

pound  C  «  ;  but  this  would  at  once  lose  the  elements 


R 

T>/ 

Q 

OH 

fR 

of  water,  and  thus  be  converted  into  an  acetone,  C  •<  R'. 

(o 

This  could  not  undergo  the  second  change  necessary  for 
the  formation  of  an  acid,  but  if  any  further  change  should 
take  place  it  would  involve  the  breaking  down  of  the 
residues. 

Finally,  it  is  clear  that  neither  the  formation  of  an  alde- 
hyde, of  an  acid,  nor  of  an  acetone  is  possible  in  the  case 
of  a  tertiary  alcohol  if  the  formulas  above  given  are  correct. 

The  first  change  which  we  should  expect  to  take  place 

f  R 

|    -pr 

on  the  oxidation  of  a  tertiary  alcohol,  C  •{  T>,,  ,  would  be 

[OH 

the  breaking  down  of  one  of  the  residues. 

Similar  changes  take  place  in  a  number  of  other  cases. 
Thus  the  phosphines  and  mercaptans  undergo  change  in 
the  same  general  way,  but  in  these  cases  the  phenomena 
are  complicated  by  the  fact  that  the  valency  of  phosphorus 
and  of  sulphur  is  greater  toward  oxygen  than  toward 
hydrogen.  The  analogy  between  the  three  cases  will  be 
clear  from  the  following  considerations  : 

1TT 

XI  f      C\TT 

H  \ 

TJ,  is  changed  to  carbonic  acid,  C  <   OH  ; 

H  ^° 

1T> 
H  \  "^ 

TT,  is  changed   to  an  acid,  C  •<  OH; 
H  ^° 

f  TT  f  OH 

\  '  OH- 

Phosphine,  P  \  H,  is  changed  to  phosphoric  acid,  P  -j   QJJ  ' 

H 


COMPOUNDS  OF  CARBON.  157 

A  derivative 
of  phos- 
phine,      P  •<  H,  is  changed  to  a  phosphinic  acid,  P 

0 
OTT 


O 
R 


fR 

•<  H,  i 

t-H 

OH- 

Q  ' 

A   mercap-       /-  T> 

tan,  S  \  TT,  is  changed  to  a  sulphonic  acid,  S 

O 

Ethereal  Salts.  —  In  general  when  acids  and  bases  act 
upon  each  other  salts  are  formed,  water  being  eliminated. 
So,  also,  when  alcohols  and  carbon  acids  act  upon  each 
other  compounds  similar  to  salts  are  formed,  water  being 
eliminated  :  — 

H.COOH  +    C2H5.OH    =  H.CO.OC2H5  +  H20; 

Formic  acid.  Alcohol.  New  compound. 

/OH  /OCH3 

SO2(          4-   2CH3.OH    =    SO2(  +  2H2O; 

XOH  XOCH3 

Sulphuric  acid.         Methyl  alcohol.  New  compound. 

NQ2—  OH  +  C2H5—  OH  =  NO2—  OC2H5  +  H.O. 

Nitric  acid.  Alcohol.  New  compound. 

It  will  be  seen  that  these  compounds  differ  from  salts  in 
that  they  contain  hydrocarbon  residues  in  the  place  of 
metals.  Salts  were  defined  as  acids  in  which  a  base  residue 
is  substituted  for  the  hydrogen  of  the  hydroxyl  group. 
These  compounds  are  acids  in  which  a  hydrocarbon  residue 
is  substituted  for  the  hydrogen  of  the  hydroxyl  group.  All 
compounds  of  this  kind  are  called  ethereal  salts.  The 
analogy  between  ethereal  salts  and  ordinary  salts  is  very 
close,  and,  if  the  nature  of  the  latter  is  understood,  that  of 
the  former  will  also  be  clear.  There  are  ethereal  salts  de- 
rived from  monobasic,  dibasic,  tribasic  acids,  etc.,  and  there 
are  ethereal  salts  containing  univalent,  bivalen*  trivalent, 
etc.  ,  hydrocarbon  residues  ;  for  example, 

Ethereal  salts  of  monobasic  acids  :  — 

N02.OC2H5,  CH3.CO.OCH8,  etc. 

Ethyl  nitrate.  Methyl  acetate. 

8 


158    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

Ethereal  salts  of  dibasic  acids : — 

^  (  OCH3  p  TT  (  CO.OC2H5 

so*  I  OCH3>         °'H*  I  co.oc2H5 ' etc- 

Methyl  sulphate.  Ethyl  succinate. 

Ethereal  salts  of  tribasic  acids : — 

roc2H5  rco.oc2H5 

PO  \  OC2H5,          C3H5  \  CO.OC2H5,  etc. 
(OC2H5  (CO.OC2H5 

Ethyl  phosphate.  Ethyl  tricarballylate. 

The  above-mentioned  ethereal  salts  all  contain  univalent 
hydrocarbon  residues.  Among  those  containing  bivalent 
residues  may  be  mentioned : — 

CH3.C0.0 
CH8.CO.O 

Ethylene  diacetate. 

The  ordinary  fats  are  examples  of  ethereal  salts  con- 
taining a  trivalent  residue  : — 


C15HS1CO.O-)  C17HK.COO) 

015H31.CO.O  [  CSH5  and   C17H35.CO.O  [ 
C)5HS,CO.O).  C17HS5.CO.OJ 


Glyceryl  tripalmitate,  Glyceryl  tristearate, 

or  palmitin.  or  stearin. 

Experimental  Evidence. — The  fact  that  ethereal  salts  are, 
in  many  cases,  formed  by  the  direct  action  of  acids  upon 
alcohols,  and  that  water  is  formed  at  the  same  time,  taken 
together  with  the  knowledge  we  possess  regarding  the  con- 
stitution of  acids  and  alcohols,  points  clearly  to  the  consti- 
tution given  above  for  these  ethers.  But  another  method 
of  formation  furnishes  more  decisive  evidence. 

If  the  silver  salts  of  acids  are  treated  with  the  chlorides, 
bromides,  or  iodides  of  hydrocarbon  residues,  ethereal 
salts  are  formed  in  which  the  residues  plainly  occupy  the 
place  which  was  occupied  by  the  silver  in  the  salts,  and 
the  silver  itself  is  found  in  combination  with  the  chlorine, 
bromine,  or  iodine  which  was  in  combination  with  the  hy- 
drocarbon residues.  This  is  seen  in  the  following  typical 
reactions : — 

CH3.CO.OAg    +    C2H5I    =    CH3.CO.OC2H5    +    Agl. 

Silver  acetate.  Ethyl  iodide.  Ethyl  acetate. 


COMPOUNDS  OF  CARBON.  159 

/CO.OAg  /CO.OCH3 

C2H4(  +  2CH3 1  =  C2H4(  +  2AgL 

XCO.OAg  XCO.OCHS 

Silver  succinate.       Methyl  iodide.  Methyl  succinate. 

Ethers. — The  ethers  are  the  analogues  of  the  metallic 
oxides.  They  consist  of  two  hydrocarbon  residues,  united 
by  means  of  an  oxygen  atom,  just  as  the  metallic  oxides 
consist  of  two  basic  residues  united  by  means  of  an  oxygen 
atom.  Examples  of  these  are  the  following : — 
CH3  C2H5X  C2H5, 

)0,  )0,  )0,  etc. 

CH/  CH/  C2H/ 

Methyl  ether.          Methyl-ethyl  ether.          Ethyl  ether. 

Experimental  Evidence. — The  constitution  of  these  com- 
pounds is  rendered  clear  by  a  consideration  of  one  of  the 
principal  methods  for  their  formation  : — 

When  an  alcohol  is  treated  with  sodium  or  potassium, 
as  we  have  seen,  the  hydrogen  of  the  hydroxyl  is  replaced 
by  the  metal.  Compounds,  such  as  sodium  ethylate, 
C2H5.ONa,  sodium  methylate,  CH3.ONa,  etc.,  are  thus 
obtained.  If  these  compounds  are  further  treated  with 
the  iodides  of  hydrocarbon  residues,  the  iodine  combines 
with  the  metal  and  the  residues  unite : — 

C2H5.ONa  +  C2H5I  =  C2H5-O— C2H5  +  Nal, 

Sodium  ethylate.      Ethyl  iodide.  Ethyl  ether. 

CH3.ONa    +  CH3I    =    CH3— O— CH3  +  Nal, 

Sodium  methylate.    Methyl  iodide.  Methyl  ether. 

CH3.01Sra    +  C2H5I  =  CH  —  O— C2H5  +  Nal, 

Sodium  methylate.    Ethyl  iodide.       Methyl-ethyl  ether. 

C2H5.ONa  +  CH,I    =  C2H5— 0— CH3  +  Nal. 

Sodium  ethylate.       Methyl  iodide.    Methyl-ethyl  ether. 

These  reactions  clearly  show  what  the  constitution  of 
the  ethers  formed  must  be.  It  appears  that  in  each  case 
the  hydrocarbon  residue  enters  into  the  new  compound  in 
the  place  occupied  by  the  metal,  and,  according  to  our  con- 
ceptions concerning  alcohols,  this  metal  is  united  to  the 
rest  of  the  molecule  in  which  it  is  contained  by  means  of 
an  oxygen  atom. 

Anhydrides. — The  anhydrides  of  carbon  compounds  are 
derived  from  carbon  acids  in  the  same  way  that  anhy- 


160    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

drides  in  general  are  derived  from  acids ;  and  all  the  pos- 
sibilities considered  above  hold  good  for  these  anhydrides. 
There  are  anhydrides  derived  from  monobasic,  dibasic, 
tribasic  acids,  etc.  There  are  partial  and  complete  anhy- 
drides; but,  further,  there  are  anhydrides  derived  from 
compounds  which  partake  of  the  double  character  of 
alcohol  and  acid.  In  these  compounds  the  hydroxyl  which 
imparts  the  alcoholic  character,  and  that  which  imparts  the 
acid  character,  both  together  furnish  the  elements  which 
unite  to  form  the  water  given  off. 


CONSTITUTION  OF  SUBSTITUTION-PRODUCTS.     161 


CHAPTER    XI. 

CONSTITUTION   OF   SUBSTITUTION-PRODUCTS. 

WE  have  thus  far  had  to  deal  with  the  various  classes  of 
chemical  compounds  that  are  known  to  exist,  and  it  has 
been  shown  that  each  class  is  characterized  by  some  pecu- 
liarity of  constitution  that  is  recognized  in  each  member  of 
the  class.  There  is  in  each  compound  a  peculiar  grouping 
of  atoms  that  determines  its  character,  making  it  an  acid 
or  an  alcohol,  an  acetone  or  an  aldehyde^  etc.  As  long  as 
this  group  remains  unchanged,  the  compound  belongs  to 
the  same  class.  If  the  group  is  changed,  the  compound 
loses  its  characteristics  and  belongs  to  another  class.  On 
the  other  hand,  the  hydrocarbon  residues,  with  which  the 
class  groups  are  united,  may  undergo  a  variety  of  changes 
without  interfering  with  the  general  properties  of  the  com- 
pounds. The  most  common  of  these  changes  are  those 
which  are  effected  by  substitution. 

Chemical  compounds  act  upon  one  another,  in  general, 
in  two  ways :  1st.  They  unite  directly,  forming  only  one 
product,  as  in  the  following  reactions : — 

NH3        +        HC1        —        NH4C1 

Ammonia.  Ammonium  chloride. 

C2H4         +         Br2  C2H4Br2 

Ethylene.  Ethylene  bromide. 

2d.  They  exchange  certain  constituents,  forming  two  or 
more  new  products,  thus  : — 

C2H6       +      C12    =    C2H5C1      +     HC1 

Ethane.  Chlorethane. 

CH3COH    -f      6C1    =  CC13.COH   +  3HC1 

Aldehyde.  Trichloraldehyde. 

C6H6       -f  H2S04  =  C6H5.S03H  +     H2O 

Benzene.  Benzenesulphonic  acid. 

C6H.       +  HNOS  =  C,H5(N02)  +     H2O. 

Benzene.  Nitrobenzene. 


162    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

The  latter  kind  of  action  is  by  far  the  most  common.  It 
is  called  substitution.  In  the  above  examples  the  principal 
products  are  called  substitution-products,  though,  strictly 
speaking,  both  products  are  substitution-products. 

While  the  act  of  substitution  is  involved  in  nearly  all 
chemical  reactions,  and  hence  nearly  all  chemical  com- 
pounds may  be  considered  as  substitution-products  with 
reference  to  some  other  compounds,  still  it  is  customary  to 
include  under  this  head  only  those  which  are  formed  by 
the  substitution  of  atoms  or  atomic  groups  for  hydrogen 
in  carbon  compounds,  and  the  substitutions  spoken  of  are 
those  which  can  be  actually  effected  —  not  imaginary  cases. 

Substitution-products  containing  Chlorine,  Bromine,  or 
Iodine.  —  The  simplest  examples  of  substitution-products 
are  those  which  are  formed  by  the  action  of  the  so-called 
halogens  (Cl,  Br,  I)  on  carbon  compounds.  The  action 
consists  in  the  abstraction  of  one  or  more  atoms  of  hydrogen 
from  the  compound,  and,  as  is  believed,  the  filling  of  the 
places  thus  left  vacant  by  a  corresponding  number  of  atoms 
of  the  substituting  element.*  The  constitution  of  the  pro- 
ducts is  the  same  as  that  of  the  compounds  from  which  they 
are  derived.  Thus  when  chlorine  acts  upon  acetic  acid, 
CH3.CO.OH,  the  following  reactions  take  place  succes- 
sively :  — 

CH3.CO.OH  +  Cl,  =  CH2C1.CO.OH  +  HC1. 
CH,C1.CO.OH  +  C12  =  CHC12.CO  OH  +  HC1. 
CHC12.CO.OH  +  CJ2  =  CC13.CO.OH  -fc-  HC1. 

The  constitution  of  the  three  products  is  the  same  as  that 
of  the  acid  from  which  they  are  derived. 

*  This  is,  of  course,  only  a  statement  of  the  result  of  the  action. 
There  may  be,  and  probably  are,  intermediate  stages  in  which  com- 
pounds more  complex  than  those  finally  obtained  are  formed.  When 
chlorine  acts  upon  benzene,  C6H6,  the  first  product  obtained  is  chlor- 
benzene,  C6H6C1,  and  we  represent  the  action  by  the  equation  :  — 


That  tells  part  of  the  story,  and  an  important  part  no  doubt;  but 
there  is  much  left  untold,  and  for  the  good  reason  that  we  know 
nothing  about  it.  It  may  be  that  the  chlorine  first  combines  with 
the  benzene  to  form  an  unstable  addition-product,  which  afterward 
breaks  down  forming  chlorbenzene  and  hydrochloric  acid,  but  until 
such  compounds  have  been  isolated  speculation  in  regard  to  them 
would  appear  to  be  of  comparatively  little  value.. 


CONSTITUTION  OF  SUBSTITUTION-PRODUCTS.     163 

Among  the  simple  substitution-products,  however,  differ- 

ences are  possible,  and  are  actually  observed,  which  are  not 

possible  in  the  original   compounds.     Take,  for  example, 

the   compound  propane,  C,H8.     The  constitution  of  this 

H    H    H 


I      I       I 
H  —  C  —  C— 


hydrocarbon  is  H  —  C  —  C—  C  —  H.     According  to  our  fun- 


damental  conceptions  in  regard  to  constitution,  the  hy- 
drogen atoms  cannot  be  arranged  in  any  other  way  with 
reference  to  the  carbon  atoms.  There  is  only  one  hydro- 
carbon of  this  composition  possible.  But  the  carbon  atoms 
in  the  compound  differ  from  one  another.  The  two  which 
are  represented  in  the  formula  as  ending  the  chain  are 
alike,  while  the  central  atom  differs  from  them.  The  first 
are  in  combination  with  carbon  by  means  of  only  one  bond 
each,  while  the  central  atom  is  joined  to  carbon  by  means 
of  two  bonds.  We  should  naturally  expect,  then,  that  the 
difference  between  these  two  kinds  of  carbon  atoms  would 
cause  a  difference  between  the  hydrogen  atoms  combined 
with  them.  If  such  a  difference  exists,  then  different  pro- 
ducts must  be  obtained  according  as  a  substituting  atom  or 
group  takes  the  place  of  a  hydrogen  atom  attached  to  one 
of  the  terminal  carbon  atoms,  or  of  another  hydrogen  atom 
attached  to  the  central  carbon  atom.  Thus,  if  in  a  com 
pound  of  the  following  formula  ;  —  • 

7 

2  H   H   H    4 

1    H—  C—  C—  C—  H    6 

!      I      I 

3  H    H   H    5 


some  other  element,  such  as  chlorine,  is  substituted  for  any 
one  of  the  hydrogen  atoms,  numbered  1,  2,  3,  4,  5,  6,  the 
resulting  compound  should  in  each  case  be  the  same. 

If,  however,  the  same  element  as  in  the  first  case  be  sub- 
stituted for  one  of  the  hydrogen  atoms  numbered  7  or  8,  a 
compound  of  the  same  composition,  but  of  different  con- 
stitution, should  be  obtained.  The  formulas  of  the  two 
compounds  would  be  respectively 


164    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

CH2C1.CH2.CH3    and    CH3.CHC1.CH3. 

Thus,  it  is  seen  that  the  position  of  a  substituting  element 
must  be  taken  into  consideration  in  studying  the  consti- 
tution of  compounds.  In  connection  with  the  individual 
compounds,  which  will  be  briefly  considered  in  a  subsequent 
chapter,  the  methods  will  be  described  which  make  the 
determination  of  the  position  of  substituting  atoms  and 
groups  possible. 

Complex  Substitution- products. — Under  this  head  are 
included  all  those  products  which  are  formed  by  substitut- 
ing groups  of  atoms  for  the  hydrogen  of  a  carbon  compound 
either  partially  or  wholly.  In  accordance  with  what  has 
just  been  said  concerning  the  simple  substitution-products, 
it  is  plain  that,  in  studying  the  constitution  of  the  complex 
substitution-products,  two  things  must  be  taken  into  con- 
sideration : — 

1st.  The  constitution  of  the  substituting  group  itself, 
and, 

2d.  The  position  of  the  group  in  the  molecule  of  the 
substitution-product. 

Only  the  first  part  of  the  problem  will  be  dealt  with  here. 

Constitution  of  Substituting -groups. — The  groups  which 
we  shall  have  to  consider  are  the  following :  The  cyano- 
gen group  CN,  and  an  isomeric  group;  the  sulphonic  acid 
group  S03H ;  the  nitro  group  NO2 ;  the  nitroso  group  NO ; 
the  amido  group  NH2 ;  the  imido  group  NH;  and  a  few 
other  groups  intimately  connected  with  those  mentioned. 

Constitution  of  the  Group  CN. — That  acid  of  carbon, 
which  consists  of  a  nitrogen  atom  and  a  hydrogen  atom, 
united  with  a  carbon  atom,  viz.,  hydrocyanic  acid,  has 
already  been  referred  to.  By  appropriate  reactions  it  is 
possible  to  transfer  the  group  CN,  contained  in  hydro- 
cyanic acid,  to  other  compounds  in  such  a  way  that  it  takes 
the  place  of  hydrogen,  forming  a  substitution-product.  It 
is  expressed  by  the  formula  — C — N.  We  have  the  follow- 
ing reactions : — 

CH2C1.COOH  +    KCN    =  CH2(CN).COOH  +    KC1; 

Monochloracetic  Potassium  Cyanacetic  acid, 

acid.  cyanide. 


CONSTITUTION  OF  SUBSTITUTION-PRODUCTS.     165 
C2H4Br2       +  2KCN  =       C2H4(CN)2        +  2KC1. 

Ethylene  bromide.    Potassium  cyanide.     Ethylene  cyanide. 

Those  substitution-products,  which  consist  only  of  the 
group  — C — N  combined  with  a  hydrocarbon  residue,  are 
called  cyanides  or  nitriles. 

Other  compounds  are  known,  which  have  the  same  com- 
position as  the  nitriles,  but  a  different  constitution.  They 
are  known  as  isonitriles,  isocyanides,  or  carbamines.  The 
difference  between  the  cyanides  and  carbamines  is  believed 
to  consist  in  the  fact  that  in  the  former  the  carbon  of  the 
group  CN  is  the  linking  atom,  while  in  the  latter  the 
nitrogen  performs  this  function.  In  the  case  of  the  ethyl 
compounds,  this  difference  is  expressed  by  the  formulas 
C2H5— C— N  for  the  cyanide,  and  C2H5— N— C  for  the 
isocyanide.  These  formulas  are  based  upon  the  reactions 
of  the  two  classes  of  compounds.  The  reactions  are  as 
follows : — 

Ethyl  cyanide,  C2H5.CN,  when  treated  with  an  alkali, 
yields  propionic  acid,  C2H5.CO.OH.  The  nitrogen  is  given 
off,  and  oxygen  and  hydrogen  take  its  place.  The  carbon 
remains  united  to  the  residue  C2H5.  The  conclusion  is 
therefore  drawn  that  in  the  cyanide  the  carbon  atom  of 
the  group  CN  is  directly  united  with  the  hydrocarbon 
residue.  For,  if  it  had  not  been,  the  removal  of  the 
nitrogen  ought  to  have  caused  the  formation  of  a  product 
containing  a  smaller  number  of  carbon  atoms  than  the  cya- 
nide itself.  The  reaction  which  does  take  place  is  the  one 
considered  above,  which  gives  rise  to  the  formation  of  acids 
from  the  cyanides,  viz. : — 

C2H5CN    +     2H2O    =    C2H5.COOH     +     NH3. 

Ethyl  cyanide.  Propionic  acid. 

If  the  group  CN  were  in  combination  with  the  hydro- 
carbon residue  by  means  of  the  nitrogen  atom,  we  should 
expect  the  nitrogen  atom  to  remain  in  combination  with 
the  hydrocarbon  residue,  in  case  of  decomposition,  or  we 
should  expect  the  nitrogen  atom  to  take  with  it  the  carbon 
atom,  with  which  it  is  most  intimately  combined.  In  either 
case,  a  separation  of  the  carbon  atoms  would  be  the  result, 
and  we  should  obtain  products  containing  a  smaller  num- 
ber of  carbon  atoms  than  the  original  compound  contained. 
This  is  exactly  what  takes  place  when  the  carbamines  are 
decomposed.  When  treated  with  hydrochloric  acid,  they 


166    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

yield  two  products  ;  one  of  these  is  formic  acid,  a  compound 
containing  one  atom  of  carbon  ;  the  other  consists  of  the 
hydrocarbon  residue  of  the  original  compound  combined 
with  the  nitrogen  atom  and  hydrogen.  Thus  in  the  case 
of  ethyl  isocyanide  the  decomposition  is  represented  as 
follows  :  — 


C2H5—  N-C    +     2H20    =      H     N     +    H.COOH. 

H  J 

Ethylcarbamine.  Ethylamine.  Formic  acid. 

(Oft 

The  fact  that  the  compound  N  4  H      ,  in  which  the 

I1? 

nitrogen  atom  is  evidently  in  combination  with  the  hydro- 
carbon residue,  is  so  readily  formed,  leads  to  the  conclu- 
sion that  the  same  kind  of  union  exists  in  the  isocyanide. 
The  fact,  further,  that  one  carbon  atom  is  given  off  so 
readily  from  the  molecule,  indicates  clearly  that  it  was 
held  in  combination  in  some  manner  different  from  that  in 
which  the  other  carbon  atoms  of  the  molecule  are  held  in 
combination. 

In  the  terms  of  the  valency  hypothesis  the  difference 
between  the  cyanides  and  carbamines  is  expressed  by  the 
general  formulas  R  —  C=N  for  the  cyanides,  and  R  —  Ni=C 
or  R  —  N=C=  for  the  isocyanides. 

Constitution  of  the  Group  SO3H.  —  By  the  action  of  con- 
centrated sulphuric  acid  upon  hydrocarbons  and  various 
other  compounds  containing  hydrogen  derivatives  are  ob- 
tained which  differ  from  the  original  compounds  in  con- 
taining the  group  SO3H  in  the  place  of  hydrogen.  The 
reaction  consists  in  the  formation  of  water  and  the  new 
derivative,  thus  :  — 

OH  C8H5 


Benzene.  Benzenesul  phonic  acid. 

All  these  products  act  like  acids,  so  that  we  are  justified 
in  assuming  the  presence  of  hydroxyl  in  them.  As  they 
are  so  readily  formed  from  sulphuric  acid,  it  is  also  fair 
to  assume  that  the  group  SO2OH  is  a  residue  of  sulphuric 
acid.  Then,  if  the  constitution  of  sulphuric  acid  is  known, 
the  constitution  of  this  group  may  be  inferred.  The  fact 


CONSTITUTION  OF  SUBSTITUTION-PRODUCTS.     167 

that  it  is  a  residue  of  sulphuric  acid  is  shown  also  in  the 
following  way  :  By  replacing  one  of  the  hydroxyl  groups 
of  sulphuric  acid  by  an  atom  of  chlorine,  a  compound  of 


the  formula  SO2^          is  obtained,  which,  by  simple  treat- 

XOH 

ment  with  water,  is  reconverted  into  sulphuric  acid.  There 
can  be  little  doubt  that  the  group  SO2OH  of  this  chloride 
has  exactly  the  same  constitution  as  the  corresponding 
group  of  the  acid.  But,  if  this  chloride  is  allowed  to  act 
upon  benzene,  benzenesulphonic  acid  and  hydrochloric 
acid  are  the  products,  the  former  having  all  the  properties 
possessed  by  the  benzenesulphonic  acid  formed  by  the  ac- 
tion of  sulphuric  acid  on  benzene.  The  reaction  takes 
place  thus : — 

/Cl 

C6H6  +  SO2(          —  C6H5.S02.OH  +  HC1. 
XOH 

Here,  evidently,  the  group  SO2.OH  of  the  chloride  takes 
the  place  of  an  atom  of  hydrogen  in  benzene. 

Assuming  then  the  general  formula  SO2.OH  for  the 
group,  it  remains  to  decide  in  what  manner  the  atoms  of 
the  group  SO2  are  united.  The  first  question  to  be  an- 
swered, and  perhaps  the  only  one  that  can  be  answered 
by  experimental  methods  at  present,  is,  whether  in  the 
compound  containing  the  group  SO2,  as,  for  example, 
C6H5.SO2.OH,  the  sulphur  is  in  direct  combination  with 
the  hydrocarbon  or  not. 

When  the  sulphonic  acids  are  reduced,  they  yield,  as 
final  products,  the  corresponding  sulphydrates.  Thus, 
C6H5.SO3H  yields  C6H5SH.  Further,  when  the  sulphy- 
drates are  oxidized,  they  yield  sulphonic  acids :  C2H5.SH 
yields  C2H5.S03H,  etc.  The  simplest  interpretation  of 
these  facts  is  found  in  the  assumption  that  the  sulphur  is 
in  direct  combination  with  the  hydrocarbon,  as  represented 
in  the  formulas : — 

C2H  —  S— H,  C2H5— S— O2— OH,  C6H3— S- H,  and 
C6H-S-02-OH. 

It  is  impossible  to  determine  with  any  degree  of  cer- 
tainty whether  the  two  oxygen  atoms  (02)  are  both  in 
direct  combination  with  sulphur  or  not,  though  the  opinion 


168    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

is  now  commonly  held  that  in  the  sulphonic  acids  the  sul- 
phur acts  as  a  hexad  and  that  the  constitution  is  expressed 

O 

II 
by  the  general  formula  R — S— O — H.     This  question  is 


I 


intimately  connected  with  that  in  regard  to  the  constitu- 
tion of  sulphuric  acid,  and  this  will  be  discussed  further  on. 

Constitution  of  the  Group  NO2. — When  concentrated 
nitric  acid  acts  upon  hydrocarbons,  etc.,  the  group  NO2  is 
frequently  substituted  for  hydrogen,  thus: — 

C6H6    +     N02OH    -    C6H5.N02     +     H,O. 

Benzene.  Nitric  acid.  Nitrobenzene. 

The  reaction,  as  will  be  noticed,  is  similar  to  that  which 
takes  place  when  sulphuric  instead  of  nitric  acid  is  used. 
Just  as  in  the  former  case  it  may  be  assumed  that  the  group 
SO3H  is  a  residue  of  sulphuric  acid,  so  in  the  latter  case  it 
may  be  assumed  that  the  group  NO2  is  a  residue  of  nitric 
acid.  The  formula  that  we  accept  for  nitric  acid  will  show 
us  the  constitution  of  the  group  NO2.  Again,  nitro  com- 
pounds are  formed  by  treating  a  chloride,  bromide,  or 
iodide  of  a  hydrocarbon  residue  with  silver  nitrite,  AgNO2, 
the  reaction  taking  place  as  follows : — 

C2H5I     -f     AgN02    =    C2H5(N02)     +     Agl. 

Ethyl  iodide.  Nitroethane. 

It  would  appear  from  the  latter  reaction  that  the  group 
NO2  has  the  same  constitution  in  the  nitro  derivatives  that 
it  has  in  nitrous  acid ;  but  this  is  not  the  case,  or,  at  least, 
certain  facts  seem  to  indicate  clearly  a  difference  between 
the  groups. 

There  are  two  series  of  compounds  of  the  same  compo- 
sition, but  of  different  constitution,  both  of  which  contain 
the  group  NO2.  The  members  of  one  of  these  series  are 
ethereal  salts  of  nitrous  acid.  If  nitrous  acid  contains 
hydroxyl,  then  the  ethereal  salts  have  the  general  consti- 
tution R — O — NO,  in  which  R  represents  a  hydrocarbon 
residue.  The  characteristic  feature  in  the  constitution  of 
these  ethereal  salts  is  the  same  as  that  which  we  find  in 
all  ethereal  salts,  viz.,  the  acid  group  is  combined  with 
the  hydrocarbon  residue  by  means  of  an  atom  of  oxygen. 


CONSTITUTION  OF  SUBSTITUTION-PRODUCTS.     169 

That  this  is  true  of  the  ethereal  salts  of  nitrous  acid  is 
shown  by  the  fact  that  when  nascent  hydrogen  acts  upon 
them  they  yield  the  alcohols  corresponding  to  the  hydro- 
carbon residues  which  they  contain,  and  at  the  same  time 
ammonia.  If  the  nitrogen  atom  were  directly  united  with 
the  hydrocarbon  residue,  we  should  probably  find  it  in 
combination  with  this  residue  after  the  above  reduction. 
The  decomposition  that  actually  takes  place  is  represented 
thus:  — 
Ether,  R-—  0—  NO,  yields— 

R_  O—  H  and  H—  N/~ 
XH 

Alcohol.  Ammonia. 

With  the  constitution  assumed  for  the  ethereal  salt  it  is 
evident  that  the  formation  of  an  alcohol  by  the  addition  of 
hydrogen  would  necessitate  the  splitting  off  of  the  group 
containing  nitrogen. 

On  the  other  hand,  the  second  series  of  compounds  are 
not  ethereal  salts,  but  true  substitution-products.  They 
consist  of  hydrocarbon  residues  combined  with  the  group 
NO2  by  means  of  the  nitrogen  atom.  Their  general  con- 
stitution is  expressed  by  the  formula  R  —  NO2. 

This  conclusion  is  reached  by  a  study  of  the  products 
of  the  reduction  of  nitro  compounds.  When  treated  with 
nascent  hydrogen,  they  yield  products  known  as  amine 
bases,  which  have  been  shown  to  bo  ammonia  in  which  a 
hydrocarbon  residue  is  substituted  for  one  hydrogen  atom. 
The  decomposition  is  represented  thus  :  — 


-f    6H    =    R-NH2     +     2H2O. 

Nitro  product.  Amine  base. 

In  the  product  obtained  in  this  case  it  is  evident  that 
the  nitrogen  atom  is  in  direct  combination  with  the  hy- 
drocarbon residue,  and  hence  we  may  assume  that  this 
kind  of  combination  also  existed  in  the  original  nitro 
compound. 

Accepting  the  above  formula  for  nitro  compounds,  it  is 
difficult  to  see  how  they  can  be  formed  by  the  reaction 
with  silver  nitrite.  For,  if  the  hydrocarbon  residue  took 
the  place  occupied  by  the  silver  in  the  salt,  it  is  plain  that 
the  product  would  be  an  ethereal  salt,  which,  according 
to  what  has  already  been  said,  must  have  the  formula 


170    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

R — 0 — NO.  The  product,  however,  is  not  an  ethereal 
salt.  Consequently,  some  other  change  besides  that  of  an 
interchange  of  places  by  the  silver  atom  and  the  hydro- 
carbon residue  must  be  accomplished  at  the  same  time. 
Consequently,  further,  the  group  NO2  in  nitrous  acid  has  a 
constitution  differing  from  that  of  the  group  NO2  of  nitro 
compounds.* 

As  regards  the  constitution  of  these  two  groups,  in 
nitrous  acid  there  is,  in  the  first  place,  in  all  probability 
one  hydroxyl.  This  gives  one  oxygen  atom  combined 
with  hydrogen  on  the  one  hand,  and,  on  the  other,  with 
nitrogen,  thus:  H — O— N — .  The  only  thing  additional 
that  is  present  in  the  molecule  is  an  atom  of  oxygen,  which 
it  is  safe  to  suppose  is  combined  with  nitrogen;  hence 
we  have  the  group  — O — N — O  or  ( — O — N— O)  as  the 
characteristic  group  of  the  acid  and  its  derivatives.  But 
the  group  of  nitro  compounds  unites  with  residues  by  means 
of  its  nitrogen  atom,  as  has  been  seen.  Hence  we  can 

/° 

conceive  of  two  formulas  for  the  group  NO2,  viz.,  — NY 

in  which  the  nitrogen  is  trivalent,  and  — N^    ,  in  which 

the  nitrogen  is  quinquivalent.  It  has  not  been  found  pos- 
sible to  decide  which  of  these  two  formulas  is  the  correct 

one.     The  latter,   — N/^    ,  is    now  more   commonly  ac- 


*  It  has  been  suggested  that  when  an  iodide,  as,  for  example, 
ethyl  iodide,  C2H5I,  acts  upon  silver  nitrite,  AgNO2,  the  reaction 
takes  place  in  two  stages :  First,  an  addition-product  is  formed  as 
represented  in  the  equation:  — 

Ag— O— N=O  +  C2H5I  ==  Ag— O— N— O 

C2H?I ' 

Second,  this  addition-product  breaks  down  into  a  nitro  compound 
and  silver  iodide : — 


Ag_ O— N— O  =  O=N=O  +  Agl 

C2HgI  CjjHg 

Of  course,  if  this  kind^of  action  takes  place  in  one  case,  it  probably 
takes  place  in  others. 


CONSTITUTION  OF  SUBSTITUTION-PROD UCTS.    171 

cepted  than  the  former.  The  essential  difference  between 
these  formulas  is  that  in  one  the  two  oxygen  atoms  are 
represented  as  united  with  each  other,  and  in  the  other 
they  are  represented  as  only  in  combination  with  nitrogen. 
Our  experimental  methods  of  investigation  are  not  subtle 
enough  to  cope  with  this  question. 

Constitution  of  the  Group  NO. — There  are  two  classes 
of  compounds,  both  of  which  contain  a  group  NO.  They 
are  the  nitroso  and  the  isonitroso  compounds.  The  former 
can  be  prepared  by  treating  with  nitrous  acid  compounds 
which  contain  the  so-called  imido  group  NH,  or  those 
which  contain  the  group  CH.  If,  on  the  other  hand, 
compounds  which  contain  the  group  CH2  are  treated  with 
nitrous  acid,  isonitroso  compounds  are  formed.  The  fact 
that  the  nitroso  compounds  are  converted  into  nitro  com- 
pounds by  oxidation,  and  into  amido  compounds  containing 
NH2  by  reduction/points  to  an  analogy  with  these  substances. 
They  are  hence  represented  by  the  general  formula  R — NO. 

The  isonitroso  compounds  are  made  by  treating  with 
hydroxylamine,  NH3O,  compounds  which  contain  the  ace- 
tone or  aldehyde  group,  CO.  The  reaction  may  be  repre- 
sented in  two  ways,  according  as  we  assume  the  presence  of 
hydroxyl  in  hydroxylaraine,  or  not : — 


/H 
X*— CiO  +  HHiHNO  =  X— C( 

XNO  +  H20 


X— CIO  +  H2iNOH  =  X— C— N— OH  +  H2O. 

• 

The  results  of  experiments  on  the  subject  indicate  that 
in  the  isonitroso  compounds  the  relations  between  the 
elements  are  expressed  by  the  formula  X — C — N — OH, 
or  X— C— N — OH.  A  simple  example  of  the  isonitroso 
compounds  is  furnished  by  acetoxime,  which  is  formed  by 
the  action  of  hydroxylamine  on  acetone,  as  represented  in 
the  equation : 

*  X  simply  represents  whatever  the  carbonyl  CO  group  is  in 
combination  with. 


172    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 
CH3  CH3 

CO  +  H2NOH  ==  C=N—  O—  H  -f  H2O. 
CH 


The  aldoximes  which  are  formed  by  the  action  of  hy- 
droxylamine  on  aldehydes  are  also  isonitroso  compounds.  A 
simple  example  is  that  formed  from  ordinary  aldehyde  :  — 

CH3  CH3 

CO     +     H2NOH  C=N—  O—  H     +     H2O. 

H  H 

Constitution  of  the  Group  NH2.  —  Compounds  containing 
the  group  NH2  are  called  amido  compounds,  or  primary 
amines.  The  group  is  plainly  a  residue  of  ammonia  and  is 

/H 
univalent,  having  the  constitution  —  N^      .     These  com- 

H. 
pounds   are  readily  obtained  by  the  action   of  nascent 

hydrogen  on  nitro  derivatives,  the  group  NO2  being  con- 
verted into  NH2.  Amido  substitution-  products  have  prop- 
erties similar  to  those  of  ammonia,  which  fact  furnishes 
further  evidence  of  a  similarity  in  the  constitution  of  the 
two.  They  have  basic  properties  in  the  same  sense  that 
ammonia  has  basic  properties,  i.e.,  they  unite  directly  with 
acids,  forming  salts.  In  addition  to  the  above  method  of 
formation,  there  is  also  the  action  of  ammonia  upon  chlo- 
rides, bromides,  or  iodides  of  hydrocarbon  residues  :  — 

CHBr  NH      =    CH(NH  HBr. 


C2H5Br     +     NH3     =    C2H5(NH2) 

Ethyl  bromide.  Ethylamine. 


This  latter  method  indicates  very  clearly  the  intimate 
connection  between  amido  compounds  and  ammonia. 

Constitution  of  the  Group  NH.  —  Finally,  there  is  a  class 
of  compounds  called  imido  compounds,  or  secondary  amines. 
These  contain  the  group  NH,  which  is  bivalent,  and  hence 
occupies  the  place  of  two  hydrogen  atoms.  Like  amido 
compounds,  they  may  be  regarded  as  derived  from  am- 
monia by  the  substitution  of  hydrocarbon  residues  for  two 


CONSTITUTION  OF  SUBSTITUTION-PROD  UCTS.     1 73 

hydrogen  atoms.     The  constitution  of  the  group  and  of  the 
compounds  is  readily  understood.     We  have : — 

C2H5Br)  C2H5) 

I      +     NH3    =    C2H5f]Sr    +    2HBr. 
C2H5Brj  HJ 

2  mol.  ethyl  Diethylamine. 

bromide. 

Just  as  there  are  primary,  secondary,  and  tertiary  alco- 
hols derived  from  methyl  alcohol  by  the  substitution  of 
hydrocarbon  residues  for  one,  two,  or  three  hydrogen  atoms, 
so  there  are  primary,  secondary,  and  tertiary  amines  derived 
from  ammonia  by  the  substitution  of  hydrocarbon  residues 
for  one,  two,  or  three  atoms  of  hydrogen.  The  general  for- 
mulas of  the  three  classes  of  amines  are : — 

R  fR  fR 

H,  1SNR',     and    £N  R'. 

H  (H  (R" 

Primary  Secondary  Tertiary 

amine.  amine.  amine. 

Constitution  of  the  Groups  N2H3  and  N2H2. — The  hydra- 
zine  compounds  are  closely  allied  to  the  amines.  They  bear 
the  same  relation  to  hydrazine,  N2H4,  that  the  amines  bear 
to  ammonia.  Diethylhydrazine  (C2H5)2N2H2,  is  prepared 
by  starting  from  diethylamine  (C2H5)2NH.  When  this  is 
treated  with  nitrous  acid  a  nitroso  compound  (C2H5)2N(NO), 
is  formed.  By  reduction  the  nitroso  compound  is  converted 
into  the  corresponding  amido  compound  (C2H5)2N — NH2, 
which  is  diethylhydrazine. 


The  various  classes  of  chemical  compounds  which  have 
thus  been  studied  are  the  principal  classes  with  which  we 
have  to  deal.  There  are  a  few  other  classes,  among  which 
are  the  so-called  mustard  oils,  the  diazo  compounds,  and  the 
quinones.  These  will  be  taken  up  later,  in  connection  with 
better  known  compounds  with  which  they  are  most  closely 
allied. 

In  classifying  compounds  we  have  distinguished  between 
general  classes  and  their  substitution-products.     This  dis 
tinction  is  generally  justified,  though,  in  a  certain  sense, 
even  those  compounds  which  belong  to  the  general  classes 


174    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

are  substitution-products,  or,  at  least,  may  be  considered 
as  such.  This  is  particularly  the  case  with  the  compounds 
of  carbon,  all  of  which  may  be  considered  as  derived  from 
certain  hydrocarbons  by  the  introduction  of  various  groups. 
Thus  the  alcohols  are  derived  from  hydrocarbons  by  sub- 
stituting hydroxyl,  OH,  for  hydrogen;  the  acids  by  substi- 
tuting carboxyl,  CO.OH,  for  hydrogen,  etc.  Speaking, 
then,  of  all  the  groups  which  have  been  studied  as  substi- 
tuting groups,  the  general  statement  may  be  made  that : 
In  all  substituting  groups  another  or  other  elements  of  the 
same  character  can  be  substituted  for  the  characterizing 
element  or  elements.  Thus,  as  we  have  already  seen,  sul- 
phur can  be  substituted  for  oxygen.  Further,  selenium 
or  tellurium  can  be  substituted  for  sulphur;  phosphorus 
can  be  substituted  for  nitrogen,  etc.  Thus  new  compounds 
are  formed,  but  the  constitution  of  these  is  the  same  as  that 
of  the  compounds  from  which  they  are  derived,  and  hence 
they  require  no  separate  treatment  in  this  place. 


CONSTITUTION  OF  CHEMICAL  COMPOUNDS.     175 


CHAPTER    XII. 

SPECIAL   STUDY   OF   THE    CONSTITUTION   OF   CHEMICAL 
COMPOUNDS. 

FROM  what  has  been  said  in  the  preceding  chapters  the 
constitution  of  most  compounds  will  be  understood.  In 
this  chapter  the  constitution  of  individual  compounds  will 
be  treated  so  far  as  they  require  separate  treatment.  In 
this  section  the  compounds  of  carbon  will  chiefly  occupy 
our  attention,  because  more  is  known  concerning  their 
constitution  than  concerning  that  of  other  compounds, 
and  indeed  our  ideas  regarding  the  constitution  of  the  so- 
called  inorganic  compounds  are  for  the  most  part  appli- 
cations of  ideas  gained  in  the  study  of  the  compounds  of 
carbon. 


COMPOUNDS   NOT   CONTAINING   CARBON,    OR   INORGANIC 
COMPOUNDS. 

The  compounds  which  the  univalent  elements,  hydrogen, 
chlorine,  bromine,  iodine,  fluorine,  sodium,  potassium,  lith- 
ium, caesium,  rubidium,  silver,  form  with  one  another  appear 
to  have  the  simplest  constitution  of  which  we  have  any 
conception.  They  require  no  explanation  here. 

Compounds  of   Chlorine,   etc.,   with    Oxygen,   and  with 
Oxygen  and  Hydrogen. — Hydrogen  peroxide  has  the  em- 
pirical formula  H2O2.     If  in  this  compound  the  oxygen  is 
bivalent,  the  simplest   arrangement  of  the  atoms  is  that 
represented  in  the  formula  H — O — O — H.    There  is  in- 
deed no  independent  evidence  of  the  correctness  of  this 
|  formula,  and  there  is  some  evidence  in  favor  of  the  view 
ithat  in  this  compound  the  oxygen  acts  as  a  quadrivalent 
element,*  as  indicated  in  the  formula  H — O=O — H. 

*  The  existence  of  the  oxide  of  silver  of  the  formula  Ag4O,  and  of 
the  compound  of  methyl  ether  and  hydrochloric  acid,  (CH3)2O.HC1 

or  rro3  /O\  m>  furnishes  evidence  of  the  power  of  oxygen  to  act 
C.H.3  /      \  v« 

as  a  tetra^ . 


176     PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

The  acids  which  chlorine  forms  with  oxygen  and  hydro- 
gen have  already  been  treated  incidentally  in  connection 
with  the  subject  of  valency.     It  was  shown  that  the  most 
probable   view  is  that  in  all   these   acids   except   hypo- 
chlorous  acid,  HOC1,  the  chlorine  is  polyvalent.     Thus, 
chlorous  acid  is  believed  to  be  correctly  represented  by 
the  formula  0=01 — O — H,  in  which  the  chlorine  is  triv- 
O 
II 
alent,  chloric   acid  by  Cl — O — H,  in  which  the  chlorine 

O 

is   quinquivalent,   and   perchloric    acid    by   the   formula 
O 

O— Cl — O — H,  in  which  the  chlorine  is  septivalent.     As 

has  been  pointed  out,  these  acids  are  regarded  as  derived 
from  hydroxyl  compounds  by  the  elimination  of  water. 
A  large  number  of  salts  of  periodic  acid  are  known  which 
cannot  be  readily  explained  on  any  other  assumption  but 
that  they  are  derived  from  a  number  of  acids  which  in 
turn  are  derivatives  of  the  acid  I(OH)7,  mainly  of  the 
acid  OI(OH)5  or  O=I(OH)5  in  which  the  iodine  appears 
to  be  septivalent. 

Compounds  of  Sulphur,  etc.,  with  Oxygen,  and  with 
Oxygen  and  Hydrogen. — Sulphur  forms  a  number  of  com- 
pounds with  oxygen  and  hydrogen,  some  of  which  have 
been  carefully  studied.  The  compounds  with  oxygen  alone 
are  sulphur  dioxide,  SO2,  and  sulphur  trioxide,  SO3.  With- 
out indulging  in  any  unnecessary  speculation,  the  simplest 
view  in  regard  to  these  compounds  is  that  in  them  sulphur 
is  respectively  quadrivalent  and  sexivalent.  This  view  is 
represented  by  the  following  formulas:  0— S=O  and 
O 

II 

O— S=0.  For  this  there  is  just  as  good  reason  as  for 
the  view  that  in  carbon  dioxide  the  carbon  is  quadrivalent 
as  represented  by  the  formula  O— C— O.  If  it  could  be 
shown  that  in  these  oxides  of  sulphur  the  oxygen  atoms 
are  in  combination  with  one  another,  the  above  formulas 


CONSTITUTION  OF  CHEMICAL  COMPOUNDS.    177 

would  of  course  have  to  be  abandoned,  But  this  has  not 
been  shown,  nor  is  there  the  slightest  reason  for  making 
the  assumption.  The  periodicity  of  valency  which  has 
been  discussed  makes  the  view  that  sulphur  is  sexivalent 
toward  oxygen  appear  highly  probable. 
The  acids  of  sulphur  are  the  following : — 

H2SO3,  sulphurous  acid. 

H2SO4,  sulphuric  acid. 

H2S2O7,  pyrosulphuric  acid. 

H2S2O3,  thiosulphuric  acid. 

H2S2O4,  hyposulphurous  acid. 

H2S2O6,  dithionic  acid. 

H2S3O6,  trithionic  acid. 

H2S4O6,  tetrathionic  acid. 

H2S5O6,  pentathionic  acid. 

Sulphurous  Add,  H2SO3. — This  acid  is  not  known  except 
in  water  solution,  though  a  large  number  of  its  metallic 
and  ethereal  salts  have  been  made.  From  a  study  of 
these  derivatives  conclusions  have  been  drawn  concerning 
the  constitution  of  the  acid  itself.  It  is  possible  to  con- 
ceive of  two  arrrangements  of  the  atoms  composing  sul- 
phurous acid,  both  representing  dibasic  acids.  One  of 

H 

these  arrangements   is  this,  O— S — O — H,  in  which  the 

sulphur  is  represented  as  sexivalent;    the   other  is  this, 
O 

H — O — S — O — H,  in  which  the  sulphur  is  represented  as 
quadrivalent.  The  evidence  points  clearly  to  the  first  for- 
mula. First,  the  acid  forms  two  different  salts  of  the 
formula  NaKSO3.  This  would  not  be  possible  if  it  had 
the  constitution  represented  by  the  second  formula,  in  which 
plainly  each  of  the  two  hydrogen  atoms  bears  the  same 
relation  to  the  molecule.  Again,  the  sulphonic  acids  are 
easily  made  by  treating  the  salts  of  sulphurous  acid  with 
halogen  substitution-products  of  the  hydrocarbons,  as  for 
example : — 

C2H5I     +     NaSO3H     =     C2H5.8O3H     +     Nal. 


178    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

Further,  it  has  already  been  shown  (see  Sulphonic 
Acids,  ante)  that  in  the  sulphonic  acids  the  sulphur  is  in 
all  probability  in  direct  combination  with  the  hydrocarbon 
residues.  This  makes  it,  in  turn,  appear  probable  that  in 
the  sulphite  used  in  the  above  reaction  the  metal  is  in 
direct  combination  with  sulphur.  Finally,  starting  with 
a  salt  of  a  sulphonic  acid,  as  C2H5.SO2.ONa,  ethyl  can  be 
substituted  for  the  metal,  and  thus  a  compound  obtained 

C,H, 

which  probably  has  the  constitution  O=S — O — C2H5 ;  and, 

ii 

o 

.Cl 

on  the  other  hand,  starting  with  the  oxy chloride  O=S^      , 

XC1 
and  treating  this  with  ethyl  alcohol,  a  product  is  obtained 


which    probably   has   the   constitution    O— S 


\ 


0-C2H5 


This  is  isomeric  with  the  product  obtained  from  the  sul- 
phonic acid.     The  evidence  is  therefore  in  favor  of  the 
view  that  sulphurous  acid  has  the  constitution — 
H 


=S—  O—  H 


O=S—  O—  H      or 


Sulphuric  Acid,  H2S04. — Reactions  which  have  been 
discussed  make  the  presence  of  two  hydroxyl  groups 
in  sulphuric  acid  appear  probable.  We  thus  have  the 

.OH 
formula  SO./         .     We  cannot,  however,  by  experiment 

XOH 

determine  the  constitution  of  the  group  SO2.  It  is  now 
commonly  believed  to  have  the  constitution  expressed 
by  the  formula  O— S=0,  and  sulphuric  acid  itself  is  re- 

0-H 


garded  as  O=S— O,  in  which  the  sulphur  is  sexivalent. 
-H 


CONSTITUTION  OF  CHEMICAL  COMPOUNDS.     179 


This  formula  can  be  tested  with  reference  to  one  point, 
and  that  is  as  to  whether  both  hydroxyl  groups  bear  the 
same  relation  to  the  molecule.  If  they  do,  it  should  not 
be  possible  to  prepare  two  isomeric  ethereal  salts  contain- 
ing two  different  hydrocarbon  residues.  If  two  ethereal 

CH3 
salts  of  the  composition  SO2,  ,  for  example,   could 

S0,Hf 

be  prepared,  this  would  furnish  evidence  in  favor  of  the 
view  that  sulphuric  acid  is  unsymmetrical,  or  that  the  two 
hydroxyls  bear  different  relations  to  the  molecule.  This 
view  would  be  expressed  by  a  formula  of  this  kind, 


, 

S(  or   O=S(  .      When    sulphuric 

\q_O-OH  XO—  OH 

acid  is  treated  with  phosphorus  pentachloride  under  proper 
conditions  one  hydroxyl  is  replaced  by  chlorine  and  a 

>d 
compound  of  the  formula  SO2(          is  formed.    When  this 

XOH 

is  treated  with  alcohols  ethereal  salts  are  formed.  With 
methyl  alcohol  and  ethyl  alcohol,  for  example,  the  reac- 
tions represented  in  the  following  equations  take  place  :  — 

.01  /OCH3 

SO2(          +     CH3OH   =   SO2(  +     HC1; 

XOH  XOH 

/Cl  /OC,H5 

and  SO2(  +     C2H5OH  =   SO2(  +     HC1. 

XOH  XOH 

When  the  compounds  thus  formed  are  treated  with 
phosphorus  pentachloride  the  hydroxyl  is  replaced  by 
chlorine,  and  the  products  are  represented  by  the  formulas 

OCH3  OC2H5 

SO2(  and  SO2(  .     Finally,  when   these   are 

XC1  XC1 

treated  with  alcohols,  neutral  ethereal  salts  of  sulphuric 
acid  are  formed.  If  the  former  are  treated  with  ethyl 
alcohol  and  the  latter  with  methyl  alcohol,  the  reactions 
represented  in  the  following  equations  take  place  :  — 

/OCH3  /OCH3 

S02(  +  C2H5OH  =  S02(  +  HC1; 

XC1  XOC2H6 


180    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

/OC2H5  /OC9H5 

and  S02(  +  CH3OH  =  SO/  +    HC1. 

XC1  XOCH3 

It  may  fairly  be  assumed  that  the  methyl  and  ethyl 
groups  in  one  of  these  compounds  occupy  the  same  places 
relatively  to  the  molecule  that  the  ethyl  and  methyl  groups 
do  in  the  other ;  or  the  group  OCH3  in  the  first  compound 
and  the  group  OC2H5  in  the  second  occupy  the  place  of 
the  same  hydroxyl  of  sulphuric  acid;  and  the  same  is 
probably  true  of  the  group  OC2H5  in  the  first  compound 
and  the  group  OCH3  in  the  second.  If,  then,  there  is  any 
difference  between  the  two  hydroxyls  of  sulphuric  acid,  the 
two  compounds  should  be  isomeric ;  whereas,  if  there  is  no 
difference,  the  two  hydroxyls  bear  the  same  relation  to  the 
molecule,  and  the  two  compounds  should  be  identical. 

Experiment  has  shown  that  there  is  no  difference  between 
the  ethereal  salts  prepared  as  above  described,  and  hence 
the  conclusion  is  drawn  that  the  constitution  of  sulphuric 
acid  should  be  represented  by  the  formula : — 

O.      /0-H  /0-OH 

>(  or        S( 

O^   X0— H  XO— OH 

The  question  whether  the  hydroxyls  are  in  combination 
with  sulphur  or  with  oxygen  can  be  answered  with  a  fair 
degree  of  certainty  by  the  following  considerations:  By 
replacing  the  two  hydroxyls  by  chlorine  the  compound 
Cl — SO2 — Cl  is  formed.  When  this  compound  is  treated 
with  hydrocarbons  under  certain  conditions  hydrocarbon 
residues  are  substituted  for  the  chlorine  atoms,  compounds 
called  sulphones  being  formed,  which  are  represented  by 
formulas  like  these,  CH3— SO2— C2H5,  C6H5— SO2— C6H5. 
By  reduction  these  compounds  lose  all  their  oxygen  and 
are  converted  into  sulphides,  of  which  the  following  are 
examples : — 

CH3 — S — C2H5,        C6H5 — S — C6H5. 

It  cannot  be  doubted  that  in  the  sulphides  the  hydro- 
carbon residues  are  in  combination  with  sulphur.  When 
these  sulphides  are  oxidized  they  are  transformed  into  the 
sulphones.  It  appears,  therefore,  that  in  the  compound 
Cl — SO2 — Cl  the  two  chlorine  atoms  are  in  combination 
with  the  sulphur,  and  from  this  the  conclusion  appears  to 


CONSTITUTION  OF  CHEMICAL  COMPOUNDS.     181 

be  justified  that  in  sulphuric  acid  the  two  hydroxyls  are 
in  combination  with  sulphur.     This  leads  to  the  formula 
0-H 


O— S=O,  in  which  the  sulphur  is  represented  as  sexiv- 
>— H 


0 


•=u 

H 

O        , 
alent.      As   between  this  formula  and  this        /S'         , 

0X   XOH 

in  which  the  sulphur  is  represented  as  quadrivalent,  we 
have  no  means  of  deciding.  The  fact  that  sulphur  appears 
to  be  sexivalent  toward  oxygen,  as  shown  in  the  compound 
SO3,  makes  the  formula  for  sulphuric  acid  in  which  the 
sulphur  is  represented  as  sexivalent,  the  most  probable. 

The  simplest  view  in  regard  to  sulphuric  acid  is  that 
it  is  derived  from  the  maximum  hydroxide,  S(OH)6,  by 
elimination  of  water.  The  stable  form  of  the  acid  is  that 
in  which  two  atoms  of  hydrogen  are  contained.  On  the 
other  hand,  there  are  a  number  of  salts  of  sulphuric  acid 
known  which  are  derived  from  a  more  complicated  acid 
than  ordinary  sulphuric  acid.  Examples  are  the  follow- 
ing: K3HS208,  K4H2S3O12,  and  KH3S2O8.  The  first  and 
third  may  be  regarded  as  derived  from  the  acid  S2O4(OH)4, 
the  second  from  an  acid  S3O6(OH)6.  The  first  acid  is  re- 
lated to  the  maximum  hydroxide  of  sulphur,  as  shown  in 
this  formula :— 

(H0),08/    >SO(OH)r 

If  the  hexahydroxide  loses  one  molecule  of  water,  the 
result  is  a  compound  SO(OH)4.  From  this  the  above  acid 
may  be  formed  as  thus  represented  : — 

/OIH    HOk 

(HO)K0|=^>^      I! 

The  relation  between  sulphurous  and  sulphuric  acids 
is  similar  to  that  between  formic  and  carbonic  acids,  as  is 
shown  by  the  formulas  here  given  : — 

9 


182    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

fH  fOH  (H  fOH 

C^OH  C^OH  S^OH  S^OH 

(o  (o  (o2  to, 

Formic  acid.          Carbonic  acid.          Sulphurous  Sulphuric 

acid.  acid. 

Pyrosulphuric  Acid,  H2S2O7. — Pyrosulphuric  acid  is 
formed  from  sulphuric  acid  by  the  abstraction  of-one  mole- 
cule of  water  from  two  molecules  of  the  acid  : — 


SO2: 


OH 
OH 
,OH 


SO, 


-      H20      = 


SO2 

S09 


HO 


HO 


XOH 

2  mol.  sulphuric' acid.  Pyrosulphuric  acid. 

Of  course,  the  groups  SO2  contained  in  this  compound 
must  be  regarded  as  having  the  same  constitution  as  in 
sulphuric  acid. 

Thiosulphuric  Acid,  H2S2O3. — This  acid  is  usually  con- 
sidered as  sulphuric  acid  in  which  one  of  the  hydroxyl 
groups  has  been  replaced  by  the  group  SH,  thus : — 

/OH  /SH 

S02(  S02(       . 

XOH  XOH 

Sulphuric  acid.  Thiosulphuric  acid. 

This  formula  is  rendered  probable  by  the  fact  that  the 
acid  can  be  obtained  from  sulphuric  acid  by  treating  the 
latter  with  phosphorus  sulphide.  The  action  of  the  latter 
reagent  generally  consists  in  substituting  sulphur  for  oxygen. 
The  oxygen  of  the  hydroxyl  group  is,  in  general,  more 
susceptible  to  the  influence  of  reagents  than  that  contained 
in  the  group  SO2. 

Further,  thiosulphuric  acid  is  obtained  by  allowing 
hydrogen  sulphide  to  act  upon  sulphur  trioxide,  just  as 
sulphuric  acid  is  obtained  by  allowing  water  to  act  upon 
sulphur  trioxide : 

SO2.O      +      H2O      =      SO,/ 

XOH 


CONSTITUTION  OF  CHEMICAL  COMPOUNDS.     183 


SCLO 


H2S       =      SO 


OH 


Dithionic  Acid,  H2S2O65  is  considered  as  related  to  pyro- 
sulphuric acid,  thus : — 

/OH  /H 

S02(  S02(_ 


so2( 

XOH 

Dithionic  acid. 


so2( 

XOH 

Pyrosulphuric  acid. 

Trithionic  Acid,  H2S3O6,  may  be  considered  as  derived 
from  pyrosulphuric  acid  in  the  same  way  that  thiosulphuric 
acid  is  derived  from  sulphuric  acid,  thus : — 

,SH 


S09 


,OH 
:0     : 


S02 
SO0 


O 


XOH 

Pyrosulphuric  acid. 

Or,  it  may  be  that  trithionic  acid  has  this  constitution — 


XOH 

Trithionic  acid. 


S02 


\ 


SH 
SH 


so2( 


2  mol.  thiosulphuric  acid. 


SO2 
S02 


OH 

S    - 
OH 


H2S. 


Trithionic  acid. 


This  view  finds  some  support  in  the  fact  that  trithionates 
are  formed  when  double  salts  of  thiosulphuric  acid  are 
boiled  with  water.  The  reaction  takes  place  according  to 
this  equation : — 

2AgKS203    =    Ag2S     +     K2S306. 

,OK 

,OK 


S02 


SO, 


X 


SAg 


\ 


OK 


S02 
S02 


:S 
OK 


Ag2S. 


184    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 


When  potassium  trithionate  is  boiled  with  potassium 
sulphide  potassium  thiosulphate  is  formed  :  — 

K2S306    +     K2S  2K2S203. 

This  reaction  also  indicates  an  intimate  relation  between 
thiosulphuric  and  trithionic  acids. 

Tetrathionic  Acid,  H2S4O6,  is  obtained  by  the  action  of 
iodine  on  sodium  thiosulphate,  thus  :  — 


N* 

2  mol.  sodium 
thiosulphate. 


NaS203 

Sodium  tetrathionate. 


SO, 


SO, 


.ONa  1 

'SNa 
.SNa 


ONa 


S02( 


S02( 


ONa 


+     2NaI 


ONa 


Hence,  the  formula  of  the  acid  from  which  the  latter  salt 
is  derived  is  accepted  for  tetrathionic  acid. 

Pentathionic  Acid,  H2S5O6,  is  obtained  by  treating 
barium  thiosulphate  with  sulphur  dichloride,  SC12.  The 
latter  compound  has  the  constitution  Cl — S — Cl ;  conse- 
quently, the  reaction  is  most  readily  interpreted  as  fol- 
lows : — 


S0? 


S02 


om 
SH 

SH 


XOH  f 

2  mol.  thiosul'- 


Cl— S— Cl    = 


S02 

so, 


\ 


.OH 

:S3 
OH 


2HC1. 


Pentathionic  acid. 


phuric  acid. 

The  corresponding  acids  of  selenium  and  tellurium,  as 
far  as  they  are  known,  are  represented  by  similar  formulas, 
except  selenious  acid,  which  appears  to  be  symmetrical  as 

,O— H 


represented  by  the  formula  O— Se 


0-H 


CONSTITUTION  OF  CHEMICAL  COMPOUNDS.     185 

Compounds  of  Nitrogen  with  Oxygen,  and  with  Oxygen 
and  Hydrogen. — Nitrogen  forms  with  oxygen  the  following 
compounds : — 

N2O,  nitrous  oxide  or  nitrogen  monoxide. 

NO  or  N2O2,  nitric  oxide. 

N2O3,  nitrogen  trioxide. 

NO2  or  N2O4,  nitrogen  peroxide. 

N2O5,  nitrogen  pentoxide. 

Practically  nothing  is  known  about  the  arrangement  of 
the  atoms  in  these  compounds. 

Nitrous  oxide  is  usually  represented  by  the  formula 

\  /  .     Possibly  it  is  analogous  to   chlorine   monoxide, 

O 

C12O,  in  which  the  chlorine  is  regarded  as  univalent,  The 
formula  above  given  is  not  an  expression  of  any  fact 
known  to  us,  except  the  general  one  that  nitrogen  fre- 
quently acts  as  a  trivalent  element,  and  rarely,  if  ever,  as 
a  univalent  element. 

If  nitrogen  is  trivalent,  it  is  plain  that  nitric  oxide  is 
unsaturated.  The  ease  with  which  it  takes  up  oxygen 
and  chlorine  is  in  accordance  with  the  view  that  it  is  un- 
saturated. 

There  is  little,  if  any,  experimental  evidence  in  regard 
to  the  constitution  of  nitrous  and  nitric  acids  except  of  a 
general  kind.  Nitrous  acid  probably  contains  hydroxyl, 
and  its  constitution  is  expressed  by  the  formula 
O— N — O — H,  in  which  the  nitrogen  is  trivalent.  The 
acid  may  be  regarded  as  derived  from  the  hydroxide 
N(OH)3  by  loss  of  one  molecule  of  water,  as  represented 
thus : — 

roH  co 

N^OH       =       N]  +       H20. 

(OH  (OH 

In  the  same  way  nitric  acid  is  regarded  as  derived  from 
the  maximum  hydroxide,  N(OH)5,  as  represented  thus : — 

f  OH 

|  OH  CO 

N  <{  OH       =       N  ]  O          +       2H2O. 

OH  (OH 

OH 


186     PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

While  most  salts  of  nitric  acid  are  derived  from  the  acid 
HO.NO2,  there  are  some  which  can  only  be  explained  on 
the  assumption  that  they  are  derived  from  the  acid 

r  o 

I     /"VTJT 

N  -I  QJJ,  and  the  maximum  hydroxide  or  normal  acid. 

[OH 

The  probabilities  for  ordinary  nitric  acid  are  decidedly  in 
favor  of  the  formula  in  which  the  nitrogen  is  represented 
as  quinquivalent. 

Hydroxylamine,  H3NO. — This  substance  conducts  itself 
in  many  respects  like  ammonia,  and  hence  is  regarded  as 
a  substituted  ammonia.  It  is  probable  that  its  constitu- 

(H 
tion  is  properly  expressed  by  the  formula  N  <  H    .     The 

(_  OH 

formation  of  the  aldoximes  and  acetoximes  by  treating 
aldehydes  and  acetones  with  hydroxylamine  is  fairly 
good  evidence  in  favor  of  the  view  that  hydroxylamine 
contains  hydroxyl,  for  these  products  contain  hydroxyl, 
as  has  already  been  stated. 

Compounds  of  Phosphorus  with  Oxygen,  and  with  Oxygen 
and  Hydrogen. — Phosphorus  forms  two  oxides,  viz. ; — 
P2O3,         phosphorus  trioxide. 
P2O5,         phosphorus  pentoxide. 

In  the  former  the  phosphorus  is  regarded  as  trivalent, 
and  in  the  latter  as  quinquivalent. 

There  are  several  acids  of  phosphorus,  viz. : — 
H3PO2,      hypophosphorous  acid. 
H3PO3,      phosphorous  acid. 
H3PO4,      phosphoric  acid. 
H4P2O7,     pyrophosphoric  acid. 
HPO3,       metaphosphoric  acid. 

Hypophosphorous  Acid,  H3PO2,  is  monobasic,  and  hence 
only  one  hydroxyl  group  is  assumed  as  present  in  its  mole- 

H 

cule.    This  gives  the  formula  H2PO.OH  or  O=P— O— H 

H 

in  which  the  phosphorus  atom  is  quinquivalent. 


CONSTITUTION  OF  CHEMICAL  COMPOUNDS.    187 

Phosphorous  Add,  HgPO^ — In  regard  to  the  constitu- 
tion of  this  acid  two  views  are  held.      According  to  the 

/OH 
first,  the  formula  of  the  acid  is  P— OH,  the  phosphorus 

XOH 

being  trivalent.     According  to  the  second,  the  formula  is 
0-H 

O— P — H,  the  phosphorus  being  quinquivalent.     If  the 

O— H 

former  view  is  correct,  the  acid  ought  to  be  tribasic.  In 
most  of  its  salts,  however,  it  is  only  dibasic.  Still,  ethereal 
salts  are  known  which  are  evidently  derived  from  a  tribasic 
acid,  as,  for  instance,  PO3(C2H5)3;  and,  further,  it  has 
been  shown  that  a  salt  of  the  acid  exists  in  which  there 
are  three  atoms  of  a  univalent  metal  in  the  molecule. 
These  facts  can,  however,  be  explained  as  readily  by  the 
aid  of  the  one  formula  as  the  other.  There  is  no  good 
reason  known  why  the  hydrogen  in  combination  with 
phosphorus,  PH,  should  not  have  acid  properties  as  well  as 
hydrogen  in  combination  with  nitrogen.  The  fact  that 
phosphorous  acid  is  produced  simply  by  treating  phos- 
phorus trichloride  with  water  is  in  accordance  with  the 
formula  P(OH)S.  We  have 


\ 


H 

Cl  +  H 
Cl        H 


HO  /OH 


HO  =  POH  +  3HC1. 
HO  XOH 


Though  it  is  quite  possible  to  conceive  of  the  formation  of 

/H 
a  compound  of  the  formula  OP^  ,  by  this  action. 

\OH), 

On  the  other  hand,  the  following  facts  speak  clearly  for 
O— H 


the  formula  O=P— H:— 


When  benzene  is  treated  with  phosphorus  trichloride, 
under  appropriate  conditions,  the  following  reaction  takes 
place : — 

PCla    +    C6He    =    PC12(C6H5)     +     HC1. 


188    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

When  the  main  product,  phosphenyl  chloride,  is  treated 
with  water,  the  chlorine  is  eliminated,  and  a  compound  of 
the  composition  PO2H2(C6H5)  is  formed.  The  formula  of 
this  compound  may  be  either 

O— H 

/OH  | 

1,  P^-OH  ;  or  2,  O=P— H. 


If  the  latter  is  the  formula,  then  we  may  conclude  that 
the  constitution  of   phosphorous    acid   is  similar,  i.   e., 
O — H 


O— P— H. 
O— H 


If  formula  1  is  correct,  then,  by  the  action  of  phosphorus 
pentachloride  upon  the  compound,  the  following  reaction 
ought  to  take  place  :  — 

I.  P(OH)2C6H5     +     2PC15    =    PC12(C6H5)     + 

2POC13    +     2HC1. 

If,  however,  formula  2  is  correct,  then,  under  the  same 
conditions,  the  following  reaction  would  take  place  :  — 

II,  OPH(OH)C6H5      +      2PC15    =    OPC12(C6H5)     + 

POC1  PC1  2HC1. 


In  the  former  case,  phosphenyl  chloride,  PC12(C6H5). 
would  be  formed;  in  the  latter,  phosphenyl  oxychloride, 
POC12(C6H5).  Experiment  shows  that  phosphenyl  oxy- 
chloride, phosphorus  oxychloride,  and  phosphorus  tri- 
chloride are  formed,  and  consequently  the  formula 
OPH(OH)C6H5  appears  to  be  correct;  and  phosphorous 

O-H 

acid  by  analogy  is  OPH(OH)2  or  O^P—  H. 

O—  H 

The  evidence  in  regard  to  the  constitution  of  phosphorous 
acid  is  therefore  conflicting. 

Phosphoric  Acid,  H3PO4.  —  This  acid  is  distinctly  tri- 
basic,  and  hence  three  hydroxyl  groups  are  assumed  to  be 


CONSTITUTION  OF  CHEMICAL  COMPOUNDS.     189 

present  in  it.  From  this  the  formula  PO(OH)3  follows 
directly.  The  question  still  remains  whether  the  phos- 
phorus is  quinquivalent  or  trivalent  in  the  acid.  In 

O 

||     OH 
the  former  case  the  formula  is  P^          ;    in   the   latter, 


XOH 

O] 


)H 

P^-OH       .     Phosphoric  acid  is  obtained  by  treating 

XOH 

phosphorus  oxychloride  with  water.     If  the  oxychloride  is 
O 


v      ,  then  we  should  expect  phosphoric  acid  to  have 
|XC1 
Cl 
the  first  of  the  two  formulas  given.     If  the  oxychloride 

/0-C1 

is  P— Cl       ,  then  the  acid  has  probably  the  latter  of  the 
XC1 

O  O 

II  /OH  ||     Cl 

two  formulas  given.     The  formulas  P'  and  P' 

|  XOH  XC1 

OH  Cl 

are  usually  accepted,  though  without  proofs.  Just  as  the 
acids  of  sulphur,  nitrogen,  and  chlorine  are  best  understood 
if  regarded  as  derived  from  hydroxides,  so  phosphoric 
acid  is  to  be  regarded  as  derived  from  the  maximum 
hydroxide  P(OH)5.  The  change  necessary  is  represented 
in  the  equation  : — 


PO(OH)3      +      H20,  or 


OH 


The  maximum  hydroxide  or  normal  acid  of  phosphorus 
breaks  down  by  loss  of  water  until  a  compound  is  obtained 
in  which  the  number  of  hydrogen  atoms  present  corre- 

9* 


190    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 


sponds  to  the  hydrogen  valency  of  phosphorus.  By  similar 
processes  pyrophosphoric  and  metaphosphoric  acids  are 
obtained  from  the  ordinary  acid. 

The   formation   of  pyrophosphoric  acid   is  represented 
thus  :— 


(0 

P  ^OH 

(OH 

>   + 

H20,  or 

roH 

P^OH 

to 

2H3PO4    =    H4P2O7       -f     H2O. 

The  relations  between  hypophosphorous,  phosphorous, 
and  phosphoric  acids  are  apparently  like  those  between 
formic  and  carbonic  acids,  and  those  between  sulphurous 
and  sulphuric  acids : — 

H  (OH 


f* 

J    TT 


O=P  1  H 

(OH 

Hypophosphorous 


OH 


Phosphorous 
acid. 


O—P^OH 

(OH 

Phosphoric 
acid. 


Pyrophosphoric  Acid,  H4P2O7. — This  is  a  partial  anhy- 
dride of  phosphoric  acid  formed  by  abstracting  one  mole- 
cule of  water  from  two  molecules  of  the  acid,  thus : — 


/ 

/ 

P< 


OH 

OH 
OH 
OH 


H2O 


0 


2  mol.  phosphoric  acid. 


OH 
P<OH 


Pyrophosphoric  acid. 


The  constitution  is  readily  understood  by  the  aid  of  the 
general  remarks  on  the  subject  of  anhydrides. 


CONSTITUTION  OF  CHEMICAL  COMPOUNDS.     191 

Metaphosphoric  Acid,  HP03. — The  composition  of  this 
acii  is  similar  to  that  of  nitric  acid,  HNO3.  It  is,  like 
pyrophosphoric  acid,  a  partial  anhydride  of  phosphoric 
acid,  formed  by  abstracting  one  molecule  of  water  from 
one  molecule  of  the  acid,  thus : — 

/OH 
POf  OH     —     H2O      =     PO2— OH. 

OH 

Phosphoric  acid.  Metaphosphoric  acid. 

The  formation  of  metaphosphoric  add  from  ordinary 
phosphoric  acid  is  represented  thus : — 

H3P04  HP03        +        H20,or 

f° 

=     P]0  +        H20. 

Us*  /  nir 

OH 

Metaphosphoric  acid  bears  the  same  relation  to  phos- 
phorus that  nitric  acid  bears  to  nitrogen. 


Arsenic,  antimony,  and  bismuth  form  compounds  anal- 
ogous to  those  of  phosphorus  here  described.  Much  that 
has  been  said  in  regard  to  the  latter  probably  holds  good 
for  the  former. 

Compounds  of  Boron  ivith  Oxygen  and  with  Oxygen  and 
Hydrogen. — Boron  forms  only  one  oxide,  viz.,  B2O3,  known 
as  boron  trioxide.  The  acid  to  which  it  corresponds  is  the 
maximum  hydroxide  or  normal  acid,  B(OH)3.  When 
normal  boric  acid,  B(OH)3,  is  heated  for  some  time  at  100°, 
it  loses  a  molecule  of  water,  and  the  compound  BO2H  is 
formed  thus: — 
/OH 

=      BO— OH      +      H2O. 


When  this  compound  is  heated  to  a  much  higher  tem- 
perature it  is  converted  into  the  oxide,  B2O3,  thus  :-^ 

BO— OH)  EO. 

)o      + 

BO— OH)  EQ/ 


192    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

Other  forms  of  boric  acid  are  obtained  by  similar  pro- 
cesses. Thus,  the  acid  from  which  the  most  common  salt 
of  boric  acid,  borax,  is  derived  has  the  formula  H2B4O7- 
Its  relation  to  normal  boric  acid  is  indicated  by  this  equa- 
tion : — 

5H207  or  structurally, 


4B(OH)3 

=  H2BA  +  t 

(OH 

B^OH 

(OH 

B{° 

(OH 

o 

B^OH 

(OH 

BJOH 

(OH 

f   / 

B^OH 

B    OH 

(OH 

\0 

(OH 
B^OH 

^ 

(OH 

5H20. 


Compounds  of  Silicon  with  Oxygen  and  with  Oxygen  and 
Hydrogen. — Silicon  bears  a  close  analogy  to  carbon  in 
some  respects.  It  usually  acts  as  a  quadrivalent  element, 
as  is  seen  in  the  compounds  SiH4,  SCJ4,  SiO2,  etc.  When 
the  chloride  is  treated  with  water,  we  should  expect  as  a 
product  Si(OH)4,  which  may  be  considered  as  normal  silicic 
acid.  It  appears  to  be  possible  to  obtain  this  acid,  but  it 
is  very  unstable.  It  loses  water  easily,  and  yields  an  acid, 
Si03H2,  thus:— 

Si(OH)4      =      SiO3H2      +       H2O. 

This  compound  is  usually  called  silicic  acid,  as  many  of 
the  silicates  are  derived  from  it. 

When  heated,  this  acid  yields  complicated  polysilicic  acids. 
They  are  formed  by  the  union  of  two  or  more  molecules 
of  silicic  acid  and  the  abstraction  of  varying  amounts  of 
water.  Examples  of  such  polysilicic  acids  are : — 

Si203(OH)2,  SiA(OH)4,  Si305(OH)2,  Si4O7(OH)2,  etc. 

Some  of  them  are  found  in  nature,  as  opal,  hydrophane, 
etc.  The  final  product  of  the  action  of  heat  on  silicic  acid 
is  silicon  dioxide,  SiO2. 


CONSTITUTION  OF  CHEMICAL  COMPOUNDS.     193 

General  Remarks  on  the  Relations  of  Ordinary  Acids  to 
the  so-called  Normal  Adds. — In  studying  the  common  in- 
organic acids  one  cannot  fail  to  be  struck  by  the  relations 
between  the  various  acids  formed  by  each  element  and  the 
hydroxides  of  the  element;  and  although  in  many  cases 
the  hydroxides  from  which  the  acids  appear  to  be  derived 
are  either  unknown  or  extremely  unstable,  there  are,  never- 
theless, many  facts  that  speak  in  favor  of  the  reality  Of 
these  relations.  To  repeat  briefly,  the  view  held  is  that  the 
acids  are  all  derived  from  hydroxides  of  the  elements-  The 
hydroxides  which  an  element  can  form  generally  corre- 
spond to  the  oxides;  their  composition  being  determined 
by  the  oxygen  valency  of  the  element.  Thus,  the  hypo- 
thetical hydroxides  of  the  members  of  the  nitrogen  family 
have  the  general  formulas  M(OH)3  and  M(OH)5.  The 
general  tendency  on  the  part  of  the  hydroxides  is  to  give 
up  water  enough  to  reduce  the  number  of  hydrogen  atoms 
in  the  molecule  to  that  which  corresponds  to  the  hydrogen 
valency  of  the  group.  Sometimes  this  goes  further,  as  is 
seen  in  the  case  of  carbon  and  nitrogen,  the  acids  of  which 
contain  two  atoms  and  one  atom  of  hydrogen  respectively. 
Both  of  these  elements,  it  will  be  observed,  belong  in  the 
first  series  in  which  oxygen  and  fluorine  belong.  The 
members  of  the  next  series  obey  the  law,  as  well  as  the 
members  of  the  fourth  and  sixth  short  periods. 

The  hydroxides  from  which  the  acids  are  derived  are 
called  normal  acids.  Thus,  normal  carbonic  acid  is  C(OH)4; 
normal  sulphuric  acid  S(OH)6;  normal  silicic  acid  Si(OH)4; 
normal  nitric  acid  N(OH)5 ;  normal  periodic  acid  I(OH)7, 
etc.  In  very  few  of  these  normal  acids  are  all  the  hy- 
drogen atoms  replaceable  by  metals.  Salts  of  the  order 
S(OM)6,  N(OM)B,  etc.,  are  not  known  except  in  the  case 
of  a  few  normal  acids.  Many  salts  of  sulphuric  acid,  how- 
ever, appear  to  be  formed  by  the  substitution  of  metal 
atoms  for  two  atoms  of  hydrogen  in  the  normal  acid.  The 
remaining  hydroxyls  are  commonly  represented  as  "  water 
of  crystallization,"  and  the  acid  from  which  the  salts  are 

OH 
derived  is  represented  by  the  formula  SO2^        .     We  have 

XOH 

no  means  of  deciding  whether  that  part  of  the  compound 
which  is  represented  as  water  of  crystallization  is  properly 
represented  in  this  way,  and  we  have  just  as  much  right 


194    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

to  represent  it,  or  a  part  of  it,  as  present  in  the  form  of 
hydroxyl  in  the  compound.  Thus,  for  example,  a  sulphate 
with  two  molecules  of  water  of  crystallization  can  with 

equal  right  be  represented  thus,  S  j  £QJJ  j*»  which  indicates, 

of  course,  the  same  composition  as  the  common  formula 
SO2(OM)2+2H?O.  Further,  there  are  many  double  salts 
of  sulphuric  acid  which  cannot  be  explained  at  all  by  the 
usual  methods,  but  are  easily  understood,  if  they  are  re- 
garded as  derived  from  the  acid  SO(OH)4.  Many  sulphates, 
too,  give  up  a  part  of  their  so-called  water  of  crystallization 
less  readily  than  the  rest.  The  formation  of  the  salt  which 
is  commonly  called  the  double  sulphate  of  magnesium  and 
potassium,  (SO4)2MgK2,  can  be  explained  by  assuming  that 

r  (OH), 
magnesium  sulphate  has  the  formula  SO  \  O\M    .     The 


action  of  potassium  sulphate  upon  it  would  then  be  repre- 
sented thus  :  — 

(OH)2 

f  (X  (  VOH)2  XX 

SO  \      >Mg  +  SO  \  =(Mg02)SO-(     \SO(OK)2  +  2H2O. 

(  o/  UQK),  xox 

That  metal  atoms  cannot  take  the  place  of  all  the  hy- 
drogen in  the  normal  acids  is  not  without  analogy.  Ordi- 
nary phosphoric  acid,  which  in  all  probability  contains 
three  hydroxyl  groups,  is  only  dibasic  toward  sodium. 

The  periodates,  which  until  recently  have  been  regarded 
as  inexplicable,  are  easily  understood  if  referred  to  the 
normal  acid  I(OH)r  Most  of  the  periodates  are  derived 
from  the  acid  IO(OH)5,  which  is  I(OH)7—  H2O.  The 

sodium  salt  is  IO  j  >Q^-  'I  ,  which  is  analogous  to  sodium 

phosphate,  PO  •!  K5  \  .     The   crystallized   acid   has  the 

composition  IO(OH)5,  usually  represented  by  the  formula 
HIO4+2H2O.  The  silver  salt  is  IO(OAg)5.  Many  per- 
iodates of  more  complicated  composition  are  known,  but 
they  can  be  understood  by  the  aid  of  the  assumption  that 
they  are  derived  from  acids  which  bear  to  ordinary  periodic 
acid  relations  similar  to  those  which  pyrophosphoric  acid 
bears  to  ordinary  phosphoric  acid,  and  the  polysilicic  acid 
to  ordinary  silicic  acid. 


CONSTITUTION  OF  CHEMICAL  COMPOUNDS.    195 

Salts. — The  constitution  of  the  most  important  acids 
having  been  explained,  that  of  the  salts,  in  general,  requires 
no  special  consideration,  for  the  salts  are,  as  a  rule,  very 
simple  derivatives  of  the  acids.  There  are  a  few  metals 
and  groups,  however,  which  have  the  power  of  forming 
peculiar  salts,  and  these  require  brief  consideration. 

Ammonium  Salts. — When  ammonia,  NH3,  acts  upon  an 
acid  a  salt  is  formed  by  direct  addition,  thus : — 

NH3        +        HC1        =  (NH^Cl 

Ammonium  chloride. 

NH3        +        HNO3     =         (NH4)NO3 

Ammonium  nitrate. 

2NH3        +        H2SO4     =        (NH4)2SO4. 

Ammonium  sulphate. 

The  salts  thus  formed  are  similar  to  the  salts  of  potas- 
sium, sodium,  etc.,  KC1,  KNO3,  K2SO4,  etc.  They  conduct 
themselves  like  true  metallic  salts.  Hence,  that  part  of 
these  compounds  which  corresponds  to  the  metal  in  metallic 
salts,  viz.,  the  group  NH4,  is  said  to  play  the  part  of  a 
metal,  and  to  it  the  name  of  ammonium  is  given.  Accord- 
ingly, the  salts  are  called  ammonium  salts.  These  have 
been  referred  to  incidentally  under  the  head  of  valency. 
It  was  shown  that  in  them  the  nitrogen  is  probably  quin- 
quivalent. The  formulas  of  the  above  salts  are,  accord- 
ingly : — 

H  H 

1/H  I/H 

N-^H ,  etc. 


I 
H 


|        N03) 


Salts  of  Copper  and  Mercury. — Copper  and  mercury  form 
70  series  of  salts,  of  which  the  following  are  examples : — 


CuCl  CuCl2 

CuBr  CuBr, 


HgCl        HgCl2 
HgN03     Hg(NO3)2 


Hg2S04     HgSO, 

The  specific  gravity  of  the  vapors  of  the  two  chlorides  of 
mercury  leads  to  the  formulas  HgCl  and  HgCl2.  According 
to  these  formulas,  mercury  is  bivalent,  and  the  compound 
HgCl  is  unsaturated.  It  has  been  suggested,  however, 
that  the  formula  of  the  chloride  HgCl  in  the  solid  condi- 


196    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

HgCl 
tion  is  Hg2CL  and  that  it  has  the  constitution   I         .     The 

HgCl 
Hg 
group  J     ,  as  well  as  the  mercury  atom  itself,  is  bivalent, 


J 
H 


and  thus  the  above  two  series  of  salts  are  explained.  The 
name  explanation  is  given  for  the  corresponding  salts  of 
copper.  It  is  impossible  to  decide  this  point  with  the 
present  evidence. 

A  large  number  of  compounds  are  known  which  are 
derived  from  salts  of  ammonium  and  contain  copper  and 
mercury.  They  seem  to  consist  of  ammonium  salts,  in 
which  a  portion  of  the  hydrogen  of  the  ammoniun  groups 
has  been  replaced  by  copper  or  mercury,  thus  :  — 
Cl 


H3K-Hg 

H3N-Hg  ' 

N02—  ONH2  , 

Cl 

Hg2 

The  first  of  these  is  formed  by  passing  dry  ammonia 
over  dry  mercurous  chloride  Hg2CJ2 ;  the  second,  known 
as  dimercuryamine  nitrate,  or  mercurius  solubilis  Hahne- 
manni,  is  the  black  powder  which  is  formed  by  adding 
ammonia  to  a  solution  of  mercurous  nitrate;  the  third, 
mercury amido  chloride,  or  infusible  white  precipitate,  is 
formed  by  adding  an  excess  of  ammonia  to  a  solution  of 
mercuric  chloride. 

Similar  compounds  are  formed  with  other  metals,  par- 
ticularly with  cobalt,  which  furnishes  a  very  large  number 
of  interesting  substances  of  this  kind.  These  are  too  com- 
plicated and  too  little  understood  to  permit  the  drawing  of 
positive  conclusions  concerning  their  constitution.  Their 
study  promises  interesting  results. 

Salts  of  Iron  and  Chromium. — Iron  and  chromium  form 
two  series  of  salts  in  regard  to  the  relations  between  which 
but  little  is  known.     In  the  ferrous  salts  the  iron  appears 
to  be  bivalent,  while  in  the  ferric  salts  it  is  trivalent : — 
FeCl2  FeCl3 

Fe(N08)2  Fe(N03)3 

FeS04  Fe2(SOJ3. 


CONSTITUTION  OF  CHEMICAL  COMPOUNDS.     197 

The  specific  gravities  of  the  two  chlorides  have  been 
determined,  and  they  appear  to  have  the  molecular  for- 
mulas Fe2Cl4  and  Fe2Cl6,  but  nothing  is  known  as  to  their 
constitution. 

The  same  general  statements  hold  good  for  the  two 
series  of  chromium  salts. 

Metal  Acids.  —  The  four  metals,  iron,  chromium,  man- 
ganese, and  aluminium,  form  hydroxides  of  the  general 
formula  MO.  OH,  which  conduct  themselves  like  weak 
acids,  forming  salts  with  some  metals.  Thus,  we  have 
A1O.OK  and  AlO.ONa,  salts  of  the  hydroxide  A1O.OH. 
These  are  derived  from  the  hydroxides  of  the  general  for- 
mula M(OH)3  by  elimination  of  one  molecule  of  water, 
just  as  the  common  inorganic  acids  are  derived  from  the 
normal  acids. 

Iron,  manganese,  and  chromium  yield  acids  of  the  gen- 
eral formula  MO4H2.  Thus,  we  have  FeO4H2,  MnO4H2, 
and  CrO4H2.  These  acids  are  analogous  to  sulphuric  acid, 
H2SO4,  and  a  close  resemblance  is  noticed  between  the  salts 
of  sulphuric  acid  and  those  of  chromic  acid,  which  is  the 
best  known  of  the  three  acids  named. 

In  the  absence  of  satisfactory  evidence  regarding  the 
constitution  of  these  acids,  the  only  formulas  which  we  are 
at  all  justified  in  using  are  those  which  have  the  general 


form  MO2  .    The  analogy  to  sulphuric  acid  makes  it 

XOH 

appear  probable  that  they  have  an  analogous  constitution. 
A  very  important  salt  of  chromium  is  that  known  as 
potassium  bichromate  or  pyrochromate.  The  formula  of 
this  salt  is  Cr2O7K2.  It  may  be  regarded  as  the  salt  of  an 
acid  which  is  analogous  to  pyrosulphuric  acid,  and  derived 
from  chromic  acid  by  the  abstraction  of  water,  thus  :  — 


Cr02 


CrO2 


/ 

XOH 
/OIL 
XOH 


CrO, 


CrO2 


OH 


OH 


2  mol.  chromic  acid.  Pyrochromic  acid. 


198     PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

^either  chromic  acid  itself  nor  pyrochromic  acid  can  be 
prepared  in  the  free  condition.  The  group  CrO2  does  not 
appear  to  be  capable  of  holding  hydroxyl  in  combination, 

/OH 
so  that  salts  of  the  formula  CrO2 — OM  are  not  known. 

An  acid  of  manganese  furnishes  salts  of  the  general  for- 
mula MnO4M.  No  positive  statement  can  be  made  in 
regard  to  the  constitution  of  this  acid,  except  that  it  is 
monobasic,  and  hence  probably  contains  one  hydroxyl 
group.  This  gives  the  formula  MnO3— OH,  but  the  group 
MnO3  remains  unexplained.  As  manganese  belongs  in 
Group  VII.,  it  is  probable  that  this  acid  is  analogous  to 
perchloric  and  periodic  acids,  and,  according  to  this  view, 
the  manganese  is  septivalent. 

Compounds  of  Uranium. — In  connection  with  the  sub- 
ject of  bases  it  was  mentioned  that  uranium  forms  a  pecu- 
liar set  of  salts  in  which  the  bivalent  group  UO2  is  re- 
garded as  taking  the  place  of  the  hydrogen  of  the  acids. 
Thus,  we  have  the  following  compounds,  UO2C12, 
UO2(NO3)2,  UO2(SO4),  etc.,  which  can  be  most  readily 
explained  by  assuming  that  in  them  the  group  UO2  acts 
like  a  bivalent  metal.  This  is  readily  understood,  if  ura- 
nium is  considered  to  be  sexivalent,  which  would  be  in 
accordance  with  its  position  in  the  periodic  system. 

Uranium,  further,  forms  salts  of  the  general  formula 
U,O7M,,  which  may  be  regarded  as  derived  from  an  acid, 

/(UOJOH 
U2O7H2  =  O^  ,  apparently  analogous  to  pyrosul- 

X(UO2)OH 
phuric  and  pyrochromic  acid. 


CONSTITUTION  OF  CARBON  COMPOUNDS.       199 


CHAPTER 

CONSTITUTION    OF   CARBON    COMPOUNDS. 

As  has  already  been  stated,  a  great  deal  more  is  known 
concerning  the  constitution  of  carbon  compounds  than  of 
those  compounds  which  do  not  contain  carbon.  Having 
considered  the  general  constitution  of  the  classes  of  com- 
pounds, it  remains  to  study  those  changes  which  the  mem- 
bers of  the  different  classes  undergo  without  losing  their 
characteristic  properties.  It  will  be  found  that  the  com- 
pounds of  carbon  can  be  divided  into  a  few  distinct  groups  ; 
that  each  of  these  groups  can  be  referred  to  some  funda- 
mental substance  of  which  all  the  other  members  of  the 
group  are  to  be  regarded  as  derivatives.  The  principal 
groups  are:  The  Marsh-gas,  or  Methane  compounds,  also 
called  Fatty  compounds;  the  Benzene  compounds,  also 
called  Aromatic  compounds ;  the  Naphthalene  compounds; 
and  the  Anthracene  compounds.  The  first  two  groups 
comprise  by  far  the  largest  number  of  carbon  compounds. 

METHANE  DERIVATIVES.     (FATTY  COMPOUNDS.) 

Compounds  derived  from  the  Hydrocarbons  CnH2n,2. 

The  constitution  of  methane  has  been  discussed  above 
(see  ante,  p.  136).  It  was  shown  that  by  the  linking  of 
carbon  atoms  to  one  another  the  possibility  is  given  of  the 
formation  of  an  homologous  series,  the  members  of  which 
differ  from  one  another  by  CH2,  or  a  multiple  of  this.  The 
following  members  of  the  series  have  been  particularly 
well  studied : — 

Methane,  CH4.  Pentane,  C5H12. 

Ethane,  C2H6.  Hexane,  C6HU. 

Propane,  C3H8.  Heptane,  C6H]6. 

Butane,  C4H10.  Octane,  C8H18. 

In  speaking  of  substitution- products  it  was  stated  that, 
according  to  the  views  now  held  concerning  constitution, 


200    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

only  one  mono-substitution-product  of  methane  can  exist. 
The  same  thing  is  true  of  the  other  substitution-products 
of  methane  in  which  more  than  one  substituting  group  is 
present.  Further,  we  can  conceive  of  only  one  variety  of 
methane  itself,  and  only  one  variety  has  ever  been  observed. 

Derivatives  of  Ethane,  C2H6. — According  to  the  prevail- 
ing views,  only  one  variety  of  this  hydrocarbon  can  exist, 
and  only  one  variety  has  been  observed.  Of  its  mono- 
substitution-products,  also,  only  one  variety  can  exist,  and 
only  one  variety  has  been  observed. 

Of  the  di-substitution-products,  however,  two  varieties 
are  possible,  as  will  be  seen  on  comparing  the  following 
formulas : — 

H    H  H    X 

II  II 

:_c-c-: 


X— C-C-X          and          H— C— C— X. 

II  II 

H    H  H    H 


In  the  first,  the  substituting  groups  are  in  combination 
with  different  carbon  atoms  :  in  the  second,  both  substitut- 
ing groups  are  in  combination  with  the  same  carbon  atom. 

A  number  of  compounds  are  known  belonging  to  the 
classes  of  which  these  are  the  general  formulas.  X  may 
represent  any  of  the  substituting  groups  with  which  we  have 
had  to  deal;  or  the  class  groups  CH2OH,  COH,  COOH,  etc. 

The  simplest  of  these  are  the  dichlorine  derivatives,  one 
of  which  is  CHC12.CH3,  and  the  other  CH2C1.CH2C1.  The 
first  is  called  ethylidene  chloride,  the  second  ethylene  chloride. 
The  constitution  of  these  compounds  is  deduced  from  the 
following  facts : — 

Ethylidene  chloride  is  formed  by  the  action  of  phos- 
phorus pentachloride  on  aldehyde.  It  has  been  shown 

H 
/ 

that  an  aldehyde  contains  the  group  — C— O.     Ordinary 
H 

/ 

aldehyde  is  CH3 — C — O  .  As  in  the  reaction  with  phos- 
phorus chloride,  the  oxygen  is  simply  replaced  by  chlorine, 
the  constitution  CH3 — CHC12  follows  for  ethylidene  chlo- 
ride. It  follows,  further,  that  CH2C1.CH2C1  must  be  the 
formula  of  ethylene  chloride. 


CONSTITUTION  OF  CARBON  COMPOUNDS.      201 

Other  compounds  closely  related  to  these  two  chlorides 
will  be  treated  under  the  heads  of  ethylene,  dibasic  acids, 
etc. 

Isomerism.  —  Two  or  more  substances  having  the  same 
composition,  but  different  properties,  are  said  to  be  isomeric. 
The  existence  of  isomeric  compounds  is  one  of  the  most 
interesting  as  well  as  important  facts  which  we  have  to 
deal  with  in  the  field  of  organic  chemistry.  Much  of  the 
work  that  has  been  done  in  chemistry  during  the  last  half- 
century  has  had  for  its  immediate  object  the  explanation  of 
cases  of  isomerism.  And  one  of  the  strongest  arguments 
in  favor  of  the  general  correctness  of  the  views  at  present 
held  in  regard  to  the  structure  of  the  compounds  of  carbon 
is  found  in  the  ease  with  which  the  innumerable  phenomena 
of  isomerism  are  explained  by  them. 

There  are  in  general  two  ways  in  which  compounds  may 
contain  the  same  elements  in  the  same  proportion  by  weight, 
and  still  have  different  properties  :  — 

1.  The  atoms  or  groups  entering  into  the  composition 
of  the  compound  may  be  arranged  differently  in  the  mole- 
cule. Thus  there  is  the  compound  ammonium  cyanate, 
CN(ONH4),  and  the  isomeric  compound  urea  CO(NH2)2. 
These  formulas  may  also  be  written  thus  :  N=C  —  O  —  NIL 


and  O=Cv  .     Compounds  which  bear  such  relations 

XNH2 
to  one  another  are  said  to  be  metameric. 

2.  Compounds  may  have  the  same  percentage  composi- 
tion, but  different  molecular  weights.  Thus  acetylene,  C2H2, 
benzene,  C6H6,  and  styrene,  C8H8,  bear  this  relation  to  one 
another.  Such  compounds  are  said  to  be  polymeric. 

Derivatives  of  Propane,  C3H8.  —  Propane  may  be  regarded 
as  a  mono-substitution-product  of  ethane,  derived  from  the 
latter  by  substituting  methyl,  CH3,  for  an  atom  of  hy- 
drogen. From  what  was  said  above,  it  will  be  seen  that 
only  one  variety  of  propane  can  exist. 

Under  the  head  of  substitution-products,  it  has  been 
shown  that  there  are  two  kinds  of  carbon  atoms,  and  con- 
sequently two  kinds  of  hydrogen  atoms  in  propane  ;  and 
hence,  further,  that  two  different  mono-substitution-pro- 
ducts can  be  obtained  from  this  hydrocarbon.  These  have 
the  general  formulas  :  — 


202    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

H   H   H  H   X    H 

X—  C—  C—  C—  H        and        H—  C—  C—  C—  H. 

H   H   H  H   H    H 

The  compounds  represented  by  the  first  formula  are 
known  as  propyl  compounds  ;  those  represented  by  the 
second  formula  as  isopropyl  or  pseudopropyl  compounds. 

The  two  alcohols,  normal  propyl,  CH3.CH2.CH2OH,  and 
isopropyl  or  pseudopropyl  alcohol,  CH3CHOH.CH3,  are 
the  starting-points  for  the  preparation  of  the  two  series  of 
isomeric  propyl  compounds.  As  the  former  is  a  primary 
alcohol,  it  follows,  from  what  has  been  said  concerning 
these  alcohols,  that  it  must  contain  the  group  CH2OH. 
This  can  only  be  the  case  if  the  hydroxyl  group  is  in  com- 
bination with  one  of  the  terminal  carbon  atoms.  Conse- 
quently, the  above  constitution  is  assigned  to  it.  By  re- 
placing the  hydroxyl  by  chlorine,  bromine,  iodine,  cyanogen, 
etc.,  corresponding  derivatives  are  obtained. 

Isopropyl  alcohol  is  obtained  from  acetone,  and,  being 
a  secondary  alcohol,  contains  the  group  CH.OH.  Its 
hydroxyl  is  in  combination  with  the  central  carbon  atom 
of  propane.  By  replacing  the  hydroxyl,  chlori,  nebromine, 
iodine,  cyanogen,  etc.,  corresponding  isopropyl  derivatives 
are  obtained. 

Derivatives  of  Butane,  C4Hj0.  —  Butane  may  be  regarded 
as  a  mono-substitution-product  of  propane  ;  consequently, 
two  varieties  must  be  possible,  one  of  which  would  have 
the  formula  :  — 

H    H    H    H 


H—  C—  C—  C—  C—  H; 


while  the  other  would  have  the  formula 
H    H    H 

II.  H—  C—  C—  C—  H 


/IN 

HHH 


CONSTITUTION  OF  CARBON  COMPOUNDS.       203 

As  a  matter  of  fact,  two  varieties  of  butane  are  known, 
viz.,  normal  butane  and  isobutane  or  trimethl-  methane.* 
The  former  has  the  constitution  represented  by  formula  I.  ; 
the  latter  that  represented  by  formula  II. 

Experimental  Evidence.  —  The  evidence  in  favor  of  the 
formula  of  normal  butane  is  the  same  in  nature  as  that 
given  for  ethane.  The  compound  is  formed  by  the  action 
of  zinc  or  sodium  on  iodo-ethane,  according  to  the  equa- 
tion :  — 

HH  HH  HHHH 

H—  C-C—  I  +  I—  C—  C—  H  +  Zn  =  H—  C—  C—  C—  C—  H  +  ZnI2. 

U       U  iUU 

Iodo-ethane.  Normal  butane. 

Of  course,  it  is  here  assumed  that  the  formula  of  iodo- 
ethane  is  known,  but  good  grounds  for  this  assumption 
have  been  presented.  Starting  with  this  formula,  we  are 
led  very  easily  to  the  above  formula  of  normal  butane. 

Trimeihyl-methane  is  obtained  from  pseudobutyl  iodide, 


the  constitution  of  which  is  known  to  be  /CI  —  CEL 


When  hydrogen  is  substituted  for  the  iodine  the  hydro- 
carbon is  the  product.     (See  Pseudobutyl  Alcohol.) 

*  The  simplest  name  for  the  members  of  the  methane  series  of 
hydrocarbons  are  those  in  which  the  members  are  all  regarded  as 
derivatives  of  methane.  Thus  :  — 

Ethane  is  called  methyl-methane, 


Propane  is  ethyl-methane, 


Normal  butane  is  propyl- methane,    C  -j  ^    . 

H 

f  CH3 
Isobutane  is  trimethyl-methane,         C 


204    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

Of  normal  butane,  two  kinds  of  simple  substitution-  pro- 
ducts are  possible.     These  have  the  general  formulas  :  — 
HHHH  HHXH 

I.  H—  C—  C—  C—  C—  X  and  II.  H—  C—  C—  C—  C—  H. 

nu        AAAA 

Of  trimethyl-methane,  there  are  also  two  kinds  possible, 
These  have  the  formulas  :  — 

H    H    H  H    X    H 

III.  X—  C—  C—  C-H  and  IV.  H—  C—  C—  C—  H  . 

A    A  A  i 


HHH  HHH 

Representatives  of  all  four  kinds  of  substitution-products 
are  known.     The  principal  of  these  are  the  alcohols  :  — 

1.  Normal  butyl  alcohol  (propyl-carbinol),* 

CH3.CH2.CH2.CH2.OH. 

2.  Secondary  butyl  alcohol  (methyl-ethyl-carbinol), 

CH3.CH.OH.CH2CH3. 

3.  Isobutyl  alcohol  (isopropyl-carbinol), 

CH 

CH.CH2,OH. 


*  The  simplest  names  for  the  alcohols  are  those  according  to 
which  they  are  regarded  as  derivatives  of  methyl  alcohol  or  car- 
binol.  Thus  :— 

fCH3 
H 


Ethyl  alcohol  is  methyl-carbinol,  C  • 

Normal  propyl  alcohol  is  ethyl-carbinol,  C 

Secondary  propyl  alcohol  is  dimethyl-carbinol,  C 


H  ' 
OH 


H  ' 

OH 

CH3 

CH3,   etc. 

H 

OH 


CONSTITUTION  OF  CARBON  COMPOUNDS.      205 

4.  Tertiary  butyl  alcohol  (trimethyl-carbinol), 

CH3 
CH3.C.OH 


From  each  of  these  alcohols  the  corresponding  chlorides, 
bromides,  etc.,  can  easily  be  obtained. 

Experimental  Evidence. — Normal  butyl  alcohol  is  ob- 
tained indirectly  from  normal  butyric  acid,  the  constitution 
of  which  is  known. 

Secondary  butyl  alcohol  is  converted  by  oxidation  into 
ethyl- methyl  ketone,  C2H5— CO— CH3.  It  is,  hence,  a 
secondary  alcohol,  and  its  constitution  is  that  expressed 
above. 

Isobutyl  alcohol  is  converted  into  isobutyric  acid  by 
oxidation.  The  constitution  of  the  acid  is  known,  and 
hence  also  that  of  the  alcohol. 

Tertiary  butyl  alcohol  is  a  tertiary  alcohol,  and  hence, 
from  what  has  been  said  regarding  these  alcohols,  it  follows 
that  it  contains  the  group  — C— O — H.  It  is  prepared 
by  treating  acetyl  chloride,  CH3.COC1,  with  zinc  methyl, 
Zn(CH3)2,  and  hence  contains  three  methyl  groups.  The 
only  formula  that  is  in  accordance  with  these  facts  is  that 
above  assigned  to  the  alcohol. 

Derivatives  of  Pentane,  C5H12. — Three  varieties  of  pen- 
tane are  possible,  according  to  the  theory.  These  have  the 
formulas : — 

1.  H3C.CH2.CH2.CH2.CH3. 

/CH3 

2.  H3C.CH2.CH/ 

H3CX/CH3 

3.  C        ,. 
H3C/XCH3 

All  three  of  these  compounds  are  known.  The  first  is 
normal  pentane;  the  second  is  dimethyl-ethyl-methane;  and 
the  third  tetramethyl-methane. 

Dimethyl- ethyl-methane  is  derived  from  ordinary  amyl 
10 


206    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

alcohol,  and  hence  has  the  same  general  constitution.  The 
evidence  for  the  constitution  of  this  alcohol  will  be  given 
below. 

Tetramethyl-methane  is  derived  from  the  iodide  of  ter- 
tiary butyl  alcohol  by  the  action  of  zinc  methyl.  The 
reaction  is  believed  to  take  place  as  follows  :  — 


/CH3 
Zn/        = 
x3  CH3 

CH3.CI/ 

XCH3 

Iodide  of  tertiary  Tetramethyl-methane. 

butyl  alcohol. 

A  great  variety  of  substitution-products  can  be  obtained 
from  the  isomeric  butanes  Theory  indicates  the  possible 
existence  of  eight  alcohols,  of  which  seven  are  known. 
These  are  :  — 

1.  Normal  amyl  alcohol  (butyl-carbinol), 

CH3  CH2.CH2.CH2.CH2.OH. 

2.  Methyl-propyl-carbinol, 

CH3.CHrCH2.CHOH.CH3. 

3.  Diethyl-carbinol, 

CH3.CH2.CHOH.CH2.CH3. 

4.  Isoamyl  alcohol  (isobutyl-carbinol), 

CH 

)CH.CH2.CH2OH. 
CH/ 

5.  Active  amyl  alcohol  (secondary  butyl-carbinol), 

CH3 
CH3.CH2.CH( 

XCH2OH 

6.  Methyl-isopropyl-carbinol, 

CH3.CHOH.CH( 


CONSTITUTION  OF  CARBON  COMPOUNDS.      207 

7.  Dimethyl-ethyl-carbinol, 

/CH3 
CHS.CH2.COH( 

XCH3 

These  alcohols  form  the  starting-points  for  the  prepara- 
tion of  corresponding  substitution-products. 

Experimental  Evidence. — Normal  amyl  alcohol  is  obtained 
from  normal  valeric  acid,  and  yields  this  acid  by  oxida- 
tion. The  constitution  of  the  acid  follows  from  its  method 
of  preparation.  (See  Normal  Valeric  Acid.) 

Methyl-propyl  carbinol  yields  methyl-propyl  ketone, 
CH3 — CO — C3HT,  by  oxidation,  and  is  formed  by  reduction 
of  this  compound. 

Diethyl-carbinol  yields  diethyl  ketone,  C2H5— CO— C2H5, 
by  oxidation. 

Secondary  Butyl-carbinol. — For  reasons  that  will  be  con- 
sidered further  on,  it  has  been  suggested  that  in  com- 
pounds which  are  optically  active,  there  is  a  carbon  atom 
linked  to  four  different  kinds  of  atoms  or  groups.  Such 
an  atom  is  called  asymmetrical.  Now,  as  secondary  butyl- 
carbinol  is  an  optically  active  compound,  it  is  believed  to 
contain  an  asymmetrical  carbon  atom.  Further,  it  is  a 
primary  alcohol,  and  hence  probably  contains  the  group 
CH2OH.  The  only  formula  in  accordance  with  these 
CH. 

facts  is  CH2OH — 0 — C2H5,  which  is  identical  with  the  one 

H 

above  given. 

Methyl-isopropy 'I- carbinol  is  formed  by  reduction  of  methyl- 


isopropyl  ketone,  CH8—  CO—  CH 


X 


CH3 


Dimethyl-ethyl-carbinol  acts  like  a  tertiary  alcohol,  and  is 
hence  believed  to  contain  the  group  COH.    The  formula 


208     PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

above  given  is  the  only  one  possible  for  a  tertiary  amyl 
alcohol. 

Derivatives  of  Hexane,  C6HU. — Five  hexanes  are  possible 
according  to  the  theory,  and  all  of  them  are  known.  These 
are: — 

1.  Normal  hexane,  CH3ICH2.CH2.CH2.CHrCH3. 

CH3 

2.  Dimethyl-propyl-methane,  OH3.CH2.CH2.CH/ 

CH3 
XCH.,.CH3 

3.  Methyl-diethyl-methane,CH3.CH( 

XCH2.CH3 

4.  Di-isopropyl  (tetramethyl-ethane), 


s  \_/J__l/^^VyJ.Av  • 

OH/  \CHS 


CH3 

5.  Trimethyl-ethyl-methane,  CHS—  C—  CH2.CH3  . 

CH3 

Experimental  Evidence.  —  Normal  hexane  is  formed  when 
normal  propyl  iodide  is  heated  with  sodium  :  — 


H3C.CH2,CH2I  4-  ICH2.CH2.CH3  2 

CH3.CH2.CH2.CH2.CH2.CH3  +  2NaI. 

Dimethyl-propyl-methane  is  formed  from  ethyl  iodide  and 
isobutyl  iodide  by  treating  the  mixture  with  sodium  :  — 

CH3 
CH3.CH2I  +  ICH2.CH( 

CH3 
' 


CH3.CH2.CH2.CH 


CH3 


X 


CH3 


Methyl-diethyl  methane  is  formed  by  treating  a  mixture 
of  methyl  iodide  and  active  amyl  iodide  with  sodium  :  — 


CONSTITUTION  OF  CARBON  COMPOUNDS.      209 


CH3I  +         JS)CH.CH2.CH8  +  2Na  = 

CH/ 


)CH.CH2.CH,  +   2NaI  . 

CH/ 

Di  isopropyl  (tetramethyl-ethane)  is  formed  by  treating 
isopropyl  iodide  with  sodium  : — 

CH3V  ,/CH3 


CHI  +  IHC 

CH3 
CH  CH3 

\CH— HC  +  2NaI . 

CH3 


Trimethyl-ethyl-methane  is  obtained  by  the  action  of  zinc 
ethyl  on  tertiary  butyl  iodide  : — 

(CH  ) 

2^  ___>LCH3'     +    Zu(C2H5)3    = 


CH/ 

Derivatives  of  Heptane,  C7H16. — Of  the  nine  hydrocarbons 
of  the  formula  C7H16,  the  existence  of  which  is  indicated  by 
the  theory,  five  are  known  up  to  the  present : — 

1.  Normal  heptane,  CH3.CH2.CH2.CH2.CH2.CH2.CHS. 

2.  Isoheptane  (dimethyl-butyl-methane), 


CH.CH2.CH2.CH2.CH3. 
CH/ 

CH2.CH3 

3.  Triethyl-methane,  CH3.CH2— CH 

CH2.CH3 
CH3 

4.  Dimethyl-diethyl-methane,  CH3— C— CH2.CH3 . 


210    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

5.  Methyl-ethyl-propyl-methane, 
CH3 

CH3.CH2— C-CH2.CH2.CH3. 

H 

A  large  number  of  alcohols  have  been  prepared,  and 
the  connection  between  them  and  the  heptanes  established. 
It  is  unnecessary  to  discuss  them  here. 


MONOBASIC  ACWS.  211 


CHAPTER   XIV. 

MONOBASIC   ACIDS,   CnH2n02,  ETC. 

THE  acids  of  this  series  may  be  regarded  as  substitution- 
products  of  the  hydrocarbons,  formed  by  substituting  car- 
boxyl,  COOH,  for  a  hydrogen  atom  of  the  latter ;  or  as 

OH 
carbonic  acid,  CO ^         ,  in  which  a  hydrocarbon  residue 

XOH 
has  been  substituted  for  one  of  the  hydroxyls,  as  repre- 

R 
sented  in  the  formula  O=C^         .     The   two   views   are 

XOH 

identical.     In  most  cases  these  acids  have  been  prepared 
by  converting  the  group  CN  of  the  cyanides  of  hydrocarbon 
residues  into  COOH.     If  the  constitution  of  the  cyanide  is 
known,  the  constitution  of  the  acid  is  readily  deduced. 
The  principal  members  of  the  series  are : — 
Formic  acid,  H.COOH. 

Acetic  acid,  CH3.COOH. 

Propionic  acid,  C2H5.COOH. 

Butyric  acid,  C3H7.COOH. 

Valeric  acid,  C4H9.COOH. 

Caproic  acid,  C5Hn.COOH. 

Of  formic  acid  and  its  substitution-products,  only  one 
variety  is  known. 

Of  acetic  acid  and  its  substitution-products,  also,  only 
one  variety  is  known. 

Propionic  Add. — With  propionic  acid  the  case  is  dif- 
ferent.    Of  the  acid  itself  only  one  variety  is  known,  but 
of  the  mono-substitution-products  two  varieties  are  known. 
The  constitution  of  the  acid  is  represented  thus : — 
H    H 
I      I 
H — C— C— COOH.     Now  it  is  plain  that,  in  this  com- 

H    H 

pound,  besides  the  hydrogen  of  the  carboxyl  group,  there 


^-/          v-' 

ii 


212    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

are  two  kinds  of  hydrogen  atoms — those  combined  with  a 
carbon  atom,  which  in  its  turn  is  in  combination  with  the 
group  CH2,  and  those  in  combination  with  a  carbon  atom, 
which  in  its  turn  is  in  combination  with  two  carbon  atoms. 
The  case  is  similar  to  that  of  propane,  of  which  two  varieties 
of  substitution- products  are  possible.  The  two  possibilities 
are  expressed  by  the  formulas : — 

H    X  H    H 

H— C— C— COOH    and    X-C  -C— COOH. 

H 

Those  which  have  the  first  formula  are  called  fit-substitu- 
tion-products ;  those  which  have  the  second  are  called 
/3-substitution-products.*  The  best-known  representativs 
of  these  two  classes  are  two  lactic  acids.  The  lactic  acids 
are  derived  from  propionic  acid  by  substituting  hydroxyl 
for  a  hydrogen  atom.  /3-iodo-propiouic  acid  is  converted 
into  hydracrylic  acid  (one  of  the  lactic  acids)  when  boiled 
with  water.  The  change  consists  in  substituting  hydroxyl 
(OH)  for  the  iodine.  Hydracrylic  acid  is  formed  from 
ethylene,  which  will  be  shown  to  have  the  formula 
CH2  CH2 

or      ||      ,  by  the  following  reactions : — 
CH2  CH2 

Ethylene  takes  up  hypochlorous  acid,  HC1O,  and  be- 

CH2OH 
comes  I  ,  ethylene-chlorhydrine.      This   product   is 

CH2C1 

CH2OH 
easily  transformed  into  ethylene  cyanhydrine,    |  ,  and 

CH2OH 

this  in  turn  into  hydracrylic  acid, 

CH,COOH 


*  In  naming  the  isomeric  substitution-products  of  the  acids,  that 
one  in  which  the  substituting  atom  or  group  takes  the  place  of  a 
hydrogen  in  combination  with  the  carbon  atom  with  which  the 
earboxyl  is  united  is  designated  as  the  a-product;  that  one  in 
which  a  hydrogen  of  the  next  carbon  is  replaced  is  called  the 
/3-product,  etc.  In  the  case  of  an  acid  of  the  formula  H3C.CH2.CH2 
COOH,  for  example,  the  a-product  is  represented  by  the  formula 
H3C.CH2CHXCOOH,  the  ^-product  by  H3C.CHXCH2.COOH,  and 
the  y-product  by  H2XC.CH2.CH.COOH. 


MONOBASIC  ACIDS. 


The  constitution  of  hydracrylic  acid  is  easily  deduced 
from  these  reactions.  As  /3-iodo-propionic  acid  is  converted 
into  hydracrylic  acid  by  boiling  it  with  water,  it  further 
follows  that  /3-iodo-propionic  acid  has  the  constitution — 


CH2I  |      | 

I  or     i— c— c— coo: 


|      | 


All  mono-substitution-products  of  propionic  acid  which 
have  been  converted  into  or  formed  by  simple  reactions 
from  /Modo-propionic  acid  are  called  p-  com  pounds,  and 
they  are  all  assumed  to  have  the  same  general  constitution. 

Having  determined  the  constitution  of  the  /3-compounds, 
that  of  the  a-compounds  follows.  It  must  be  represented 
by  the  other  possible  general  formula. 

Butyric  Acids.— Two  acids  of  the  formula  C3H7.COOH 
are  theoretically  possible  and  two  are  known.  These  are 
normal  butyric  acid,  CH3,CH2.CH2.COOH,  and  isobutyric 


CH3 


\ 


add,  )CH.COOH 


CH 


Experimental  Evidence.  —  Normal  butyric  acid  is  pre- 
pared by  introducing  the  group  C2H5  into  acetic  acid  by  a 
reaction  the  essential  part  of  which  is  represented  by  the 
equation  :  — 

CH2Na.COOH  +  C2H5I  =  C2H5.CH8.COOH  +  Nal. 

Further,  by  reduction,  normal  butyric  acid  yields  one  of 
the  two  possible  primary  butyl  alcohols.  It  has  been 
shown  that  the  other  possible  primary  butyl  alcohol  is  not 
a  derivative  of  normal  butane,  consequently  normal  butyric 
acid  must  have  a  constitution  like  that  of  normal  butane, 
and  it  has  the  formula  above  assigned  to  it. 

Isobutyric  acid  is  obtained  from  isopropyl  cyanide,  and 
this  has  been  shown  to  have  the  constitution 
H     CN  H 

H—  C  —  0  —  C—  H  . 


10* 


214    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 
From  this  the  above  formula  follows  for  isobutyric  acid. 

Valeric  Acids.  —  Four  acids  of  the  formula  C5H10O2  are 
known.     These  are  :  — 

1.  Normal  valeric  acid,  CH3.CH2.CH2.CH2.CO.OH, 

CH 

2.  Isovaleric  acid,  )CH.CH2.COOH. 

CH/ 


3.  Hydrotiglic  acid,  3'2\PR  m  OTT 
Methylethylaceticacid,       CH    / 

CH3 

4.  Trimethylacetic  acid,  CH3—  C—  COOH  . 

CH3 

Evidence.  —  Normal  valeric  acid   can  be  prepared  indi- 
rectly from  normal  butyl  alcohol  by  three  reactions  :  — 

CH3.CH2.CH2.CH2OH     +     HC1    = 

CH3.CH2.CH2.CH2C1    +    H2O. 

CH3.CH2.CH2.CH2C1     +    KCN  = 

CH3  CH2.CH2.CH2CN    -f    KC1. 

CH3.CH2.CH2.CH2CN    +    2H2O  = 

CH3.CH2.CH2.CH2.COOH   +   NH3. 

Isovaleric     acid     is     formed     from     isobutyl     iodide, 
CH 

^)CH.CH2I,  by  making  the  cyanide  and  converting 
CH3 
the  cyanogen  group  into  carboxyl. 

Trimethylacetic  acid  is  made  from  tertiary  butyl  iodide, 
CH3 

CH3  —  C  —  I  ,  through  the  cyanide. 
CHS 


MONOBASIC  ACIDS.  21 5 

Caproie  Acids. — Seven  acids  of  the  formula  C6H12O2  are 
known.  The  evidence  in  regard  to  the  structure  of  these 
is  of  the  same  kind  as  that  above  presented  for  other  acids 
of  the  series  and  need  not  here  be  repeated. 


The  other  acids  of  this  series  are  not  very  well  known. 
By  the  aid  of  the  foregoing  examples  the  method  of  deter- 
mining the  constituents  of  the  acids  will  be  readily  under- 
stood. 

Aldehydes. — Corresponding  to  every  primary  alcohol  and 
to  every  acid  an  aldehyde  is  possible.  The  constitution  of 
each  of  these  aldehydes  is  given  if  the  constitution  of  the 
alcohol  or  of  the  acid  from  which  it  is  obtained  is  known. 

The  aldehydes  are  produced  from  the  primary  alcohols 
by  partial  oxidation ;  and  from  the  acids  by  subjecting  a 
mixture  of  a  salt  of  the  acid  and  a  salt  of  formic  acid  to 
dry  distillation. 

Acetones  or  Ketones. — The  ketones  are  obtained  by  dis- 
tilling mixtures  of  two  acids.  If  the  constitution  of  the 
acid  or  acids  is  known,  that  of  the  ketone  obtained  in  each 
case  is  also  known. 

Diacid  Alcohols,  CnH2n+2O2. — The  alcohols  thus  far  con- 
sidered are  the  simplest.  They  act  like  mon-acid  bases, 
such  as  potassium  and  sodium  hydroxides,  KOH  and 
NaOH,  etc.  Corresponding  to  diacid  bases  there  are  also 
diacid  alcohols.  The  simplest  substance  of  this  kind  known 
is  ethylene  alcohol  or  glycol,  C2H4O2.  This  compound  is 
found  to  act  like  an  alcohol,  and  not  in  any  respect  like 
acids,  aldehydes,  ketones,  etc.  Between  it  and  ordinary 
alcohol  there  is,  however,  one  marked  difference.  The 
reactions  characteristic  of  ordinary  alcohol  can  be  carried 
further  with  this  substance.  Thus,  while  from  the  former 
only  one  metallic  derivative,  C2H6OK,  can  be  obtained 
with  any  one  metal,  from  ethylene  alcohol  two  such  deriva- 
tives can  be  obtained,  viz.,  C2H5O2K  and  C2H4O2K2. 

Under  the  influence  of  the  chlorides  of  phosphorus,  chlo- 
rine is  substituted  for  one  atom  of  hydrogen  and  the  one 
atom  of  oxygen  of  ordinary  alcohol,  the  product  C2H5C1 
being  formed ;  while,  according  to  the  relative  quantity  of 


216    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

the  chloride  of  phosphorus  brought  into  action,  two  dif- 
ferent chlorides  can  be  obtained  from  ethylene  alcohol, 
viz.,  C2H5OC1  and  C2H4C12.  By  oxidation  ordinary  alcohol 
yields  but  one  acid,  and  that  is  monobasic.  By  oxidation 
of  ethylene  alcohol  two  different  acids  can  be  obtained.  One 
of  these,  of  the  formula  C2H4O3,  is  monobasic,  and  it  has  the 
peculiarity  that  it  combines  in  itself  the  properties  of  an 
acid  and  an  alcohol ;  the  other,  of  the  formula  C2H2O4,  is 
dibasic. 

These  facts  can  be  best  explained  by  assuming  that  in 
ethylene  alcohol  there  are  two  hydroxyls,  or,  in  other 
words,  that  the  characteristic  grouping  of  the  hydrogen  and 
oxygen  in  ordinary  alcohol  is  twice  repeated  in  ethylene 
alcohol. 

This  conception  of  the  nature  of  ethylene  alcohol  finds 

OH 
expression  in  the  formula  C2H4( 

XOH 

This  formula  has  been  verified  by  a  synthesis,  the  prin- 
ciple of  which  is  shown  by  the  equation  : — 

OH 

C2H4Br,  +  2Ag(OH)  =  C2H4(          +  2AgBr. 

XOH 

Accepting  the  formula,  the  reactions  referred  to  can  be 
interpreted  thus : — 

/OH  /OK 

C2H4(  +     K     =     C2H4(  +     H; 

XOH  XOH 

/OK  /OK 

C2H4(  +     K    =    C2H4(  +     H; 

XOH  XOK 


/OH  /Cl  /Cl 

I4(  gives     C2H4(          and    C2H4( 

XOH  XOH  XC1 

OH  COOH  COOH 

C3H4(  gives      |  and      I 

XOH  CH.,OH  0 


Hydroxy- acetic  or          Oxalic  acid.  f     + 
glycolic  acid.  v  V^^  s\ 

Considering  the  oxidation  products,  and  recalling\na)t 
has  already  been  said  concerning   the   transformatidrf  of 


MONOBASIC  ACIDS.  21 7 

alcohols  into  acids,  we  are  led  to  the  belief  that  in  ethylene 
alcohol  there  are  two  primary  alcohol  groups,  CH2OH, 
and  that  each  of  these  in  turn  can  be  converted  into  car- 
boxyl.  In  the  light  of  these  considerations,  the  formula 
CH2OH 

|  for  ethylene  alcohol  becomes  extremely  probable. 

CH2OH 

Monohydroxy-monobasic  Acids,  CnH2aO3. — The  simple 
fatty  acids  have  the  general  formula  CnH2nO2.  There  is 
a  series  of  monobasic  acids,  closely  related  to  these,  which 
have  the  formula  CnH2I1O3.  The  simplest  of  these  is 
identical  with  the  first  product  of  oxidation  of  ethylene 
alcohol.  It  is  known  as  glycolic  or  hydroxyacetie  add, 
and  has  the  formula  C2H4O3.  It  has  the  properties  of  an 

CH2OH 
alcohol  as  well  as  an  acid.     The  formula    I  is  in 

COOH 

accordance  with  the  facts.  The  presence  of  the  alcoholic 
hydroxyl  is  shown  in  the  same  way  as  in  the  case  of 
simple  alcohols,  The  formula  suggests  the  possibility  of 
converting  the  compound  into  a  dibasic  acid  of  the  formula 
COOH 

,  a  transformation   which,  as   we   have   seen,  can 
COOH 
actually  be  effected. 

The  synthesis  of  the  acid  furnishes  further  proof  of  the 
correctness  of  the  view  expressed  by  the  formula.  It  has 
been  prepared  by  treating  chlor-  or  bromacetic  acid  with 
silver  hydroxide : — 

CH2Br  CH2OH 

|  +     AgOH    =      |  +     AgBr. 

COOH  COOH 

Bromacetic  acid.  Glycolic  acid. 

All  acids  which,  like  glycolic  acid,  can  be  regarded  as 
simple  acids  in  which  hydroxyl  takes  the  place  of  hydro- 
gen, are  known  as  hydroxy-  acids. 

OH 

Hydroxy-propionic   Acids,  C2H4^  . — These   acids 

XCOOH 


218    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

are  monohydroxyl  derivatives  of  propionic  acid.  It  has 
been  pointed  out  that  of  the  mono-substitution-products  of 
propionic  acid  there  are  two  varieties,  known  as  the  «-  and 
/?  compounds.  Accordingly,  we  sjiould  expect  the  exist- 
ence of  two  hydroxy-propionic  acids  of  the  formulas : — 


H    OH  H    H 

I  I      I 

C— CO.OH  and   HO— C— C— CO.OH. 

Ai  A  ' 


j.j. 
H-4- 


H 

These  are  the  only  ones  the  existence  of  which  is  fore- 
told by  the  theory.  Nevertheless,  no  less  than  four  com- 
pounds have  been  described  as  hydroxy-propionic  acids. 
These  are : — 

1.  a-Hydroxy -propionic    acid,    or    inactive    ethylidene- 
lactic  acid.     This  is  ordinary  lactic  acid,  obtained  by  fer- 
mentation of  milk. 

2.  Paralaetic  acid,  or  sarcolactic  acid,  also  called  ethyl- 
idenelactic  acid. 

3.  LCBVO-  lactic  acid. 

4.  Hydracrylic  acid. 

a-Hydroxy-propionic  acid  is  obtained  by  substituting 
hydroxyl  for  chlorine  in  a-chlorpropionic  acid.  It  is  also 

Cl 
made  from  the   compound  CH3.CH('        (obtained  from 

01 

aldehyde  by  treatment  with  phosphorus  pentachloride)  by 
substituting  hydroxyl  for  one  and  carboxyl  for  the  other 
chlorine  atom.  It  is  optically  inactive,  that  is,  it  has  no 
perceptible  effect  on  polarized  light. 

Paralaetic  acid  acts  towards  reagents  like  ordinary  lactic 
acid,  and  is  hence  shown  to  belong  to  the  a  series.  Unlike 
the  ordinary  acid,  however,  it  exerts  a  decided  influence 
on  polarized  light.  Lsevo-lactic  acid  also  conducts  itself 
like  an  a-acid,  but  it  turns  the  plane  of  polarization  to 
the  left,  while  paralactic  acid  turns  the  plane  to  the  right. 

The  isomerism  of  these  three  acids  cannot  be  explained 
by  our  ordinary  formulas.  A  suggestion  made  by  LeBel 
and  by  Van't  Hoff  furnishes  an  entirely  satisfactory  expla- 
nation. This  suggestion  has  reference  to  a  possible  new 
kind  of  isomerism  in  the  case  of  optically  active  substances. 
It  will  be  briefly  discussed  in  the  last  section.  Attention 


DIBASIC  ACIDS.  219 

may,  however,  here  be  directed  to  the  fact  that  ethylidene- 
lactic  acid  contains  what  has  been  called  (see  p.  207)  an 

OH 

asymmetrical  carbon  atom,  H3C — C— COOH.     It  is  upon 

H 

this  fact  that  the  explanation  of  this  peculiar  kind  of  isom- 
erism  is  based. 

Hydracrylic  acid  is  made  from  ft  iodo  propionic  acid, 
and  is  hence  a  /5-compound.  It  is  also  made  from  ethylene 
by  adding  hypochlorous  acid  and  substituting  carboxyl 
for  the  chlorine  in  the  resulting  compound,  as  has  been 
explained  (see  p.  212). 

Lactones. — The  y-  and  <*-hydroxy  acids  are  extremely 
unstable.  When  set  free  from  their  salts  they  lose  the 
elements  of  water  and  are  thus  transformed  into  neutral 
compounds  called  lactones.  As  they  have  neither  alco- 
holic nor  acid  properties,  it  is  believed  that  they  should  be 
represented  by  such  formulas  as  the  following : — 

1.  2.  3. 

C\~LT  r^rr  /^rj      /^tr  r^ir  r^TT  r^u      r^ti   r^TT  r^TT  r^TT 
v^n2.i^ri2.vvn2 ,  ori2.v_yri2.^xj.2.i^'£i2 ,  I^n3.v^ri2.^±i2.i^±i2« 

O-      -CO      O-  -CO  O CO 

Formulas  1  and  3  represent  ^-lactones,  and  formula  2  a 
<Mactone. 

The  relation  between  a  lactone  and  the  corresponding 
hydroxy  acid  is  shown  as  follows : — 

CH2.CH2.CH2  CH2.CH2.CH2 

I  I  =      I  I        +  H20. 

OH          COOH          O-       —CO 

y-Hydroxy-butyric  Butyro-lactone. 

acid. 


Dibasic    Acids,   CnH2n_2O4. —  Oxalic    acid,    C2H2O4,    or 
COOH 
|  ,  is  the  simplest  representative  of  these  acids.     It 

is  dibasic,  and  the  same  reactions  by  which  we  are  led  to 
conclude  that  carboxyl,  COOH,  is  present  in  the  mono- 


220    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

basic  acid  lead  also  to  the  conclusion  that,  in  the  dibasic 
acids,  there  are  two  carboxyls  present. 

The    second    member    of   this    series   is    malonie  acid, 

COOH 
OH2^  .    Of  each  of  these  acids  only  one  variety 

XCOOH 
is  possible. 

The  third  member  is  succinic  acid,    24 

XCOOH 

Of  this  there  should  be  two  varieties  corresponding  to  the 
two  lactic  acids,  or  the  two  series  of  mono-substitution- 
products  of  propionic  acid.  For  succinic  acid  may  plainly 
be  considered  as  propionic  acid  in  which  a  carboxyl  group 
has  been  substituted  for  a  hydrogen  atom.  The  two  suc- 
cinic acids  should  have  the  following  formulas  :  — 


H    CO.OH  H    H 

1.  H—C—C—  CO.OH  and  2.  CO.OH—  C—C—  CO.OH. 
H    H 


H    H 


The  second  formula  is  that  of  ordinary  succinic  acid,  and 
the  first  that  of  isosuccinic  acid. 

Experimental  Evidence. — Ordinary  succinic  acid  is  ob- 
tained from  /5-cyanpropionic  acid,  the  constitution  of  which 
H 


is  known  to  be  CN  —  C  —  C  —  CO.OH  ;  and  from  ethyl 


ene 


CH2.CN 
cyanide,   I  .  which  is  obtained  by  treating  ethylene 

CH2.CN 
bromide  with  potassium  cyanide. 

Isosuccinic  acid  is  obtained  from  a-cyanpropionic  acid, 

H    CN 

which  is  H—  C—  COOH  . 


u 


TRIE  ASIC  ACIDS.  221 


Triacid  Alcohols  and  Tribasic  Acids. 

Glycerol  (  Glycerin). — Only  one  alcohol  containing  three 
hydroxyl  groups  is  well  known.     This  is  glycerol.     Such 
alcohols  are  known  as  triacid  alcohols.     The  formula 
CH2OH 

of  glycerol  is  CHOH  .    This  formula  is  very  probable, 

CH2OH 

because,  as  a  result  of  a  large  number  of  observations  on 
carbon  compounds,  it  seems  to  be  a  law  that  one  carbon 
atom  cannot,  except  under  peculiar  conditions,  hold  in  com- 
bination more  than  one  hydroxyl  group.  If  this  be  true, 
the  above  formula  is  the  only  one  possible  for  glycerol. 
But,  again,  by  oxidation,  glycerol  yields  a  monobasic  acid 
containing  the  same  number  of  carbon  atoms;  and,  by 
further  oxidation,  a  dibasic  acid  also  containing  the  same 
number  of  carbon  atoms.  These  facts  show  that  the  group 
CH2OH  occurs  twice  in  glycerol.  But  if  there  are  two 
groups  CH2OH  present  in  glycerol,  then  the  formula  above 
given  must  be  correct. 

Glyceric  Acid  is  obtained  by  partially  oxidizing  gly- 
cerol. As  the  acid  contains  the  same  number  of  carbon 
atoms  as  glycerol,  it  is  assumed  that  the  oxidation  con- 
sists in  the  transformation  of  the  primary  alcohol  group 
CH2OH  into  COOH;  hence,  the  formula  of  glyceric 
CH2OH 

acid    is  CHOH  .      According    to    this,    a   dibasic   acid, 

COOH 
COOH 

I 
CHOH  ,  ought  to  be  obtained  by  oxidizing  glyceric  acid, 

COOH 

just  as  this  dibasic  acid  is  obtained  by  oxidizing  glycerol. 
This  transformation  has  been  effected. 


222    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 


More  Complex  Alcohols  and  Acids. 

Among  the  best  known  substances  of  greater  com- 
plexity analogous  to  those  thus  far  considered  are  tartar ic 
acid  and  citric  acid.  The  former  is  a  dibasic  acid  con- 
taining, in  addition  to  the  two  carboxyl  groups,  two  alco- 
holic hydroxyl  groups.  It  is,  hence,  a  dibasic  tetratomic 
acid.  It  is  dioxysuccinic  acid,  and  must  have  the  for 

CH.OH.COOH 
mula   |  .     It  is  obtained  from  dibromsuc- 

CH.OH.COOH 
cinic  acid  by  treating  the  latter  with  water,  thus — 

CHBr.COOH  CH.OH.COOH 

+  2H2O  =    |  +  2HBr. 

CHBr.COOH  CH.OH.COOH 

Dibromsuccinic  acid.  Tartaric  acid. 

As  in  the  case  of  the  hydroxy-propionic  acids,  there  are 
substances  isomeric  with  tartaric  acid,  for  which  the  com- 
monly accepted  theories  of  constitution  do  not  account. 
It  is  highly  probable  that  we  have  here  to  deal  with  con- 
ditions of  structure  similar  to  those  met  with  in  connec- 
tion with  the  hydroxy-propionic  acids.  Tartaric  acid,  like 
a-hydroxy-propionic  acid,  contains  an  asymmetrical  carbon 
OH 

atom,  H — C — COOH,  or  rather,  it  contains  two  such  atoms. 


i 


H 

Citric  acid  is  tribasic,  containing,  in  addition  to  its  three 
carboxyl  groups,  one  alcoholic  hydroxyl.     These  facts  are 


fCOOH 


represented  in  the  formula  C3H4(OH)  4  COOH 

(COOH 


In  addition  to  the  compounds  treated,  there  are  others 
still  more  complex,  and  derived  from  the  marsh-gas  hydro- 
carbons by  the  substitution  of  five  and  six  alcoholic  hy- 
droxyls,  etc.,  for  hydrogen.  Of  those  containing  five 
hydroxyls,  there  is  only  one  representative  known.  Of 
those  containing  six  substituting  groups,  however,  a  large 


CYANOGEN  COMPOUNDS.  223 

number  are  known.  Among  these  are  the  different  varieties 
of  sugars,  cellulose,  and  starch ;  and  the  acids  which  are 
derived  from  them.  Of  late  much  progress  has  been  made 
in  the  study  particularly  of  those  sugars  that  belong  to  the 
same  class  as  grape-sugar  or  glucose.  Many  new  sugars 
have  been  prepared  and  the  relations  between  them  estab- 
lished by  the  aid  of  the  conceptions  of  stereo-chemistry  as 
advanced  by  Van't  Hoff. 

An  examination  of  the  alcohols  referred  to  in  the  pre- 
ceding pages  reveals  the  fact  that  the  simplest  m on-acid 
alcohol,  CH4O,  contains  only  one  carbon  atom ;  the  simplest 
di-acid  alcohol,  C2H6O2,  contains  two;  the  simplest  tri  acid 
alcohol,  C3H8O3,  contains  three ;  the  simplest  tetracid  al- 
cohol, C4H10O4,  contains  four;  and  the  simplest  hex-acid 
alcohol  contains  six.  This  is  another  illustration  of  the 
truth  that,  except  under  peculiar  conditions,  one  carbon 
atom  can  hold  in  combination  but  one  hydroxyl. 

Cyanogen  Compounds. 

In  speaking  of  the  group  CN,  under  the  head  of  sub- 
stitution, it  was  shown  that  in  the  cyanides  the  arrange- 
ment is  probably  that  represented  by  the  general  formula 
R — C — N  (see  p.  164).  Cyanogen  itself  is  commonly 
represented  by  the  formula 

or 

J— N  feN 

The  simplest  compound  of  cyanogen  is  hydrocyanic  acid, 
which  probably  consists  of  the  group  — C — N  united  with 
hydrogen,  viz.,  H — C — N  or  H — C=N,  though  some 
facts  indicate  that  there  is  an  isomeric  acid  of  the  formula 
C— N— H  or  C=N— H  or  =C=N— H. 

A  variety  of  groups  or  other  elements,  as,  for  instance, 
OH,  SH,  NH2,  etc.,  can  be  substituted  for  the  hydrogen 
atom  of  this  acid.  Thus,  a  large  number  of  derivatives  are 
obtained  which  have  a  constitution  similar  to  that  of  the 
acid.  Thus  we  have  cyanic  acid,  HO— C — N ;  sulphocyanic 
acid,  HS — C— N ;  cyanamide,  H2N— C— N,  etc. 

It  has  already  been  shown  that  there  are  compounds 
containing  the  group  C — N —  (or  =C=N— ),  called  iso- 
cy  an  ides,  which  are  isomeric  with  the  ordinary  cyanides, 


224    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

and  evidence  for  the  formula  C — N —  has  also  been  given 
(see  ante,  p.  165). 

Mustard  Oils.  —  Sulphocyanic  acid,  HS — C — N  or 
HS— C=N,  like  other  acids,  yields  salts  and  ethereal  salts 
by  exchanging  its  hydrogen  for  metals  or  hydrocarbon  resi- 
dues. Examples  are  potassium  sulphocyanate,  KS — C — N; 
methyl  sulphocyanate,  CH3 — S— C — N,  etc.  Running 
parallel  to  the  salts  of  sulphocyanic  acid  is  a  series  of  com- 
pounds known  as  mustard  oils.  These  have  the  same  com- 
position, but  entirely  different  properties  and  constitution. 

The  simplest  representative  of  this  series  is  methyl  mus- 
tard oil,  which  has  the  constitution  expressed  by  the  for- 
mula S— C— N— CH3  or  S=C=N— CH3.  A  number  of 
similar  compounds  are  known,  one  of  which  is  allyl  mus- 
tard oil,  S— C— N— C3H5.  This  is  the  oil  obtained  from 
black  mustard  seed. 

The  evidence  in  favor  of  the  constitution  assigned  to  the 
mustard  oils  is  as  follows : — 

Ethyl  mustard  oil  is  formed  by  the  action  of  thiocarbonyl 
chloride  on  ethylamine,  thus  : — 

CSC12  +  NH2— C2H6  =  SC=N— C2H5  +  2HC1. 

It  is  also  formed  by  a  somewhat  circuitous  method. 
When  carbon  disulphide,  CS2,  is  allowed  to  act  upon 

/$£"« 

ethylamine,  N—  1         ,  the  ethylamine  salt  of  ethylsulpho- 

^H 

carbamic  acid  is  formed,  thus : — 

/ira.c,H8 

CS2    +    2(NH2.C2H5)     =    CS( 

XSH.NH2.C2H5 

By  appropriate  reactions  this  salt  is  resolved  into  ethyl- 
amine, hydrogen  sulphide,  and  ethyl  mustard  oil.  The 
decomposition  can  be  best  interpreted  as  follows : — 


OS 


£ 

TT 

H 

C,H6 

NH2. 

C,H5 

Hence  the  resulting  mustard  oil  apparently  retains  an 
atom  of  sulphur  united  with  carbon  alone,  and  the  carbon 


CARBONIC  ACID.  225 

in  turn  is  probably  also  united  directly  with  the  residue 
of  ethylamine,  — N.C2H5.  A  study  of  the  decomposition 
of  ethyl  mustard  oil  also  leads  to  the  formula  above  given. 
With  water  or  hydrochloric  acid  it  yields  ethylamine,  car- 
bon dioxide,  and  hydrogen  sulphide;  with  nascent  hydrogen 
it  yields  ethylamine,  formic  thioaldehyde,  and  hydrogen 
sulphide.  The  production  of  ethylamine  indicates  clearly 
that  in  the  mustard  oil  the  ethyl  group  is  in  combination 
with  the  nitrogen  atom ;  and  the  production  of  formic  thio- 
aldehyde, which  differs  from  formic  aldehyde,  H.COH,  only 
in  containing  sulphur  in  the  place  of  oxygen,  also  indicates 
that  in  ethyl  mustard  oil  the  sulphur  atom  is  in  combina- 
tion with  carbon.  These  results  are  embodied  in  the  for- 
mula accepted  for  the  mustard  oil. 

The  ethereal  salt  of  sulphocyanic  acid,  which  is  isomeric 
with  ethyl  mustard  oil,  conducts  itself  towards  reagents  in 
an  entirely  different  manner.  It  never  yields  ethylamine, 
but  always  a  compound  in  which  the  ethyl  group  is  in  com- 
bination with  sulphur,  as  ethyl  sulphide  or  ethyl-sulphonic 
acid ;  while  the  nitrogen  is  split  off  in  combination  with 
hydrogen  alone,  or  with  carbon,  hydrogen,  and  oxygen. 


Derivatives  of  Carbonic  Acid. 

The  salts  of  carbonic  acid  have  the  general  formula 
M2CO3.  They  are  derived  from  a  dibasic  acid,  H2CO3. 
This  acid  being  dibasic  probably  contains  two  hydroxyl 

,OK 
groups,  and  hence  we  are  led  to  the   formula   CO^ 

XOH 

for  carbonic  acid.  This  acid  is  not  known  except  in  solu- 
tion in  water.  If  the  attempt  is  made  to  prepare  it  from 
its  salts,  the  compound  CO2  is  always  obtained.  It  has 
already  been  stated  that  it  appears  to  be  a  law  that  one 
carbon  atom  cannot  hold  in  combination  more  than  one 
hydroxyl  group.  This  breaking  down  of  carbonic  acid 
into  water  and  the  oxide  is  an  indication  of  the  truth  of 
the  law.  The  acid  appears  to  be  formed  when  the  oxide 
is  conducted  into  water. 

Though  carbonic  acid  itself  is  practically  unknown,  a 
large  number  of  its  derivatives  are  well  known.  These 
are  obtained,  1,  by  substituting  elements  or  groups  for  the 


226    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

hydrogen  of  the  acid;  2,  by  substituting  elements  or  groups 
for  one  or  both  of  the  hydroxyl  groups ;  3,  by  substituting 
sulphur  for  the  oxygen.  Thus  we  obtain  first  salts  and 
ethereal  salts ;  then  compounds,  such  as  carbonyl  chloride, 

/Cl  8 

Cr-O ,    carbon    sulphoxide,     C(     ,    carbon     disulphide, 

XC1  XO 

C^     ;  and  finally  such  compounds  as  sulphocarbonic  acid, 

/8H  /O.C2H5 

CSV        ,  xanthogenic  add,  CS\  ,  etc. 

\QTT  \Q~LT 

Oil  Oil 

Among  the  most  important  derivatives  of  carbonic  acid 
is  the  amide,  urea  or  carbamide,  which  has  the  constitu- 

/NH2 
tion  expressed  by  the  formula  CO 


The  evidence  in  favor  of  this  formula  is  as  follows  :  — 
Urea  is  formed  by  the  action  of  carbonyl  chloride  on 
ammonia,  thus  :  — 


/ 
COC12   +    2NH3  =   C0(         '    +   2HC1; 

XNH2 

also  by   the  action  of  ammonia  upon  ethyl   carbonate, 
thus  :  — 

/OC2H5  NH2 

CO(  +   2NH3  =  CO(  +   2C2H60. 

XOC2H6 


The  latter  is  a  general  reaction  used  for  the  preparation 
of  acid  amides  from  the  ethereal  salts. 

Urea  has  the  power  of  uniting  with  bases,  acids,  and 
salts,  and  of  forming  with  them  crystallized  compounds. 
Instead  of  the  ammonia  residue  NH2  it  may  contain  resi- 
dues of  the  amine  bases,  as  NH.CH3,  NH.C2H5,  etc.  Or, 
again,  acid  residues,  such  as  C2H3O,  C7H5O,  etc.,  may  be 
substituted  for  one  or  more  of  the  hydrogen  atoms  of 
urea. 

A  large  number  of  compounds  are  allied  to  and  derived 
from  uric  acid.  They  have  frequently  been  the  subjects  of 
exhaustive  investigations,  and  a  formula  has  been  proposed 


CARBONIC  ACID.  227 

for  the  acid  which  is  in  perfect  accordance  with  all  the 
facts  learned.  It  is  a  weak  dibasic  acid,  but  it  does  not 
contain  two  carboxyl  groups,  for  its  formula  is  C5N4H4O3, 
while  a  compound  which  contains  two  carboxyl  groups 
must,  of  course,  contain  four  atoms  of  oxygen.  It  has  been 
shown  to  contain  two  urea  residues  differently  combined, 
each  of  which  retains  two  imide  groups,  NH.  Two  syn- 
theses of  the  acid  have  been  effected,  and  it  may  now  fairly 
be  claimed  that  its  constitution  has  been  determined. 


228    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 


UNIVERSITY 
-CALIFOJ2 

CHAPTER    XV. 

UNSATURATED   COMPOUNDS   ALLIED    TO    THE    MARSH-GAS 
DERIVATIVES. 

THE  hydrocarbons  and  their  derivatives  which  have 
thus  far  been  treated  have  the  peculiarity  in  common  that 
nothing  can  be  added  directly  to  them.  In  order  to  change 
them  something  must  first  be  removed  from  them.  When 
treated  with  strong  reagents  the  first  kind  of  action  that 
takes  place,  so  far  as  we  know,  is  substitution.  Thus, 
marsh-gas,  CH4,  and  chlorine  yield  CH3C1,  CH2C12,  etc.,  and 
not  CH4C1,  CH4C12.  Compounds  that  act  in  this  way  are 
called  saturated  compounds.  The  condition  of  saturation  is 
undoubtedly  dependent  upon  the  valency  of  the  elements, 
and,  in  the  terms  of  the  valency  hypothesis,  it  is  explained 
by  saying  that  in  the  saturated  compounds  all  the  bonds  or 
affinities  of  the  elements  are  satisfied. 

There  are  many  compounds  that  do  not  act  in  the  man- 
ner described.  When  treated  with  strong  reagents,  such  as 
chlorine,  bromine,  etc.,  they  take  up  these  elements  directly. 
In  many  cases  substitution  does  not  take  place  until  a  def- 
inite quantity  of  the  element  has  been  added.  Addition 
is  the  first  kind  of  action.  Thus,  ammonia  and  all  the 
substituted  ammonias  take  up  acids  and  other  compounds 
and  form  addition-products  :  — 


+    HC1         =    NH4C1. 
NH2CH3     +     HBr        =    NH3(CH3)Br. 
NH2CH3      +     CH3Br  NH2(CH3)2Br. 

The  cyanides  are  unsaturated,  as  is  shown  by  their  power 
to  take  up  hydrogen  directly  :  — 

HCN  +     4H  =    NH2.CH3. 

CH3.CN      +     4H          =    NH2.C2H5. 

Aldehydes  and  ketones  are  also  unsaturated,  as  is  shown 
by  their  power  to  take  up  hydrogen  :  — 


UNSATURATED  COMPOUNDS.  229 

CH3.COH        +     2H    ==    CH3.CH2OH. 
CH3.CO.CH3    -f     2H    =    CH3.CH(OH).CH3. 

Further,  there  are  many,  hydrocarbons  which  are  un- 
saturated,  taking  up  chlorine,  bromine,  iodine,  hydrochloric 
acid,  hydrogen,  etc.,  and  forming  addition-products.  The 
simplest  example  of  this  kind  is  ethylene,  which  has  the 
composition  C2HV  When  this  is  treated  with  bromine  this 
reaction  takes  place : — 

C2H4  +  2Br  =  C2H4Br2. 
With  hydrobromic  acid  this  reaction  takes  place : — 

C2H4  +  HBr  =  C2H5Br; 
and  with  hydrogen  this : — 

C2H4    +     2H     =    C2H6. 

If  bromine  is  allowed  to  act  further  upon  the  compound 
C2H4Br2,  no  more  bromine  is  added,  but  this  element  is 
substituted  for  a  part  of  the  hydrogen,  and  the  first  result 
is  a  reaction  of  this  kind : — 

C2H4Br2     +     2Br    =     C2H3Br3     +     HB.-. 

What  is  the  difference  between  saturated  and  unsaturated 
compounds  ?  This  question  has  already  been  discussed  in 
a  general  way  (see  ante,  pp.  99  and  101).  The  commonly 
accepted  explanation  of  most  unsaturated  compounds  is 
that  in  them  certain  elements  are  combined  by  more  than 
one  bond,  while  in  the  saturated  compounds  only  single 
bonds  exist ;  or,  in  other  words,  double  or  triple  linkage  of 
atoms  is  believed  to  exist  in  most  unsaturated  compounds, 
while  only  the  condition  of  single  linkage  is  found  in 
saturated  compounds.  This  explanation  is  not,  of  course, 
applicable  to  the  case  of  ammonia  and  its  derivatives, 
though  the  phenomenon  of  addition  in  this  case  appears  to 
be  of  the  same  character  as  in  the  other  cases  referred  to. 
In  the  cyanides  it  is  assumed  that  between  nitrogen  and 
carbon  there  is  triple  linkage,  as  indicated  in  the  formula 
CH3 — CEEN.  When  hydrogen  is  taken  up  this  triple 
linkage  is  changed  to  single  linkage  as  represented  thus : — 

+    4H    =    H3C-H2C-NH2, 

11 


230    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

and  the  carbon  and  nitrogen,  which  were  triply  linked, 
each  takes  up  two  atoms  of  hydrogen. 

In  the  aldehydes  and  ketones  it  is  assumed  that  the 
oxygen  is  united  to  carbon  by  double  linkage,  as  repre- 

H 
I 
sented  in  the  formulas,  CH3— C--=O  ,    CH3— C— CH3 ,  etc. 

II 

O 

When  saturation  takes  place  the  double  linkage  is  broken, 
and  single  linkage  takes  its  place,  as  represented  in  the 
following  equations : — 

H  H 

CH3— C=O        +     2H    =    CH3— C— O— H, 

H 

CH3— C— CH3    +    2H    =    CH3— CH— CH3. 

I 
OH 

In  these  cases  the  carbon  and  oxygen,  which  are  united  by 
double  linkage,  each  takes  up  one  atom  of  hydrogen. 

A  similar  explanation  is  sometimes  offered  for  the  satu- 
ration of  ferrous  chloride  and  similar  compounds.  The 
view  is  expressed  in  this  equation : — 

Cl     Cl  Cl     Cl 

?=Fe     +     2C1   -  Ci— F< 


Fe=Fe     +     2C1   =  Ci— Fe-Fe-Cl. 

I        I 
Cl     C 


I* 


Now,  turning  to  ethylene,  the  unsaturated  condition  of 
this  compound  can  be  explained  in  the  same  way,  and  the 
addition  of  hydrogen,  bromine,  etc.,  to  it  appears  to  be  a 
phenomenon  of  the  same  kind  as  those  just  considered. 
The  reactions  with  hydrogen,  hydrobromic  acid,  and  bro- 
mine are  represented  thus : — 

CH2  H  CH3 

CHa  H  CH, 


UNSATURATED  COMPOUNDS  231 

CH2  H  CH3 

II  i         j      I 

CH2  Br  CH2Br 

CH2  Br  CH2Br 

CH2  Br  CH2Br ' 

There  is  independent  evidence  that  when  bromine  is 
added  to  ethylene  one  atom  of  bromine  combines  with 
each  carbon  atom  and  not  both  with  one ;  therefore  it  ap- 
pears probable  that  in  the  other  additions  the  action  takes 
place  in  the  same  way. 

It  is  clear  that  a  satisfactory  explanation  of  what  is 
meant  by  double  linkage  cannot  be  given  until  it  is  known 
what  single  linkage  is.  The  condition  of  double  linkage  is 
generally  assumed  in  such  compounds  as  the  oxides  of  poly- 

^° 
valent  elements,  as  Ca— O,  Ba=O,  Cu— O,  C— O,  C, 


etc.,  and  many  familiar  reactions  are  explained  in  the  same 
way  as  the  cases  above  discussed.  Thus,  for  example,  the 
action  of  water  on  calcium  oxide  is  probably  a  reaction  of 
the  same  kind  as  that  which  takes  place  when  hydrobromic 
acid,  etc.,  act  upon  ethylene : — 

Ca  O— H          Ca— O— H 


II         +  =1 

O  H  O— 


H 

According  to  this,  calcium  oxide  is  an  unsaturated  com- 
pound in  the  same  sense  in  which  ethylene  is  unsaturated. 
While,  then,  we  cannot  say  that  the  condition  of  double 
or  triple  linkage  does,  as  a  matter  of  fact,  exist  in  unsatu- 
rated compounds,  still  the  hypothesis  is  convenient  and 
helpful,  and  may  be  held  tentatively  in  dealing  with  the 
phenomena  of  unsaturation. 

The  chief  arguments  in  favor  of  the  view  as  applied  to 
carbon  compounds  are  the  following : — 

1.  Among  carbon  compounds  the  condition  of  unsatura- 
tion does  not  generally  occur  except  in  cases  in  which  there 
is  the  possibility  of  double  or  triple  linkage  between  carbon 
atoms.  There  is,  for  example,  no  compound  CH2  and  none 
of  the  composition  CC12  or  CBr2  known. 

On  the  other  hand,  there  is  the  compound  CO,  which 


232    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

cannot  be  explained  by  the  hypothesis  under  discussion. 
It  cannot  be  assumed  that  carbon  is  bivalent  toward  oxy- 
gen, for  it  has  the  power  of  taking  up  additional  oxygen 
and  thus  becoming  saturated.  It  is  unsaturated,  but  ap- 
parently in  a  different  way  from  ethylene.  The  existence 
of  two  oxides  of  carbon  and  only  one  hydrogen  compound 
with  one  carbon  atom  in  the  molecule  is  a  fact  of  the  same 
kind  as  that  noticed  in  connection  with  sulphur,  phosphorus, 
nitrogen,  chlorine,  and  many  other  elements,  each  of  which 
forms  but  one  compound  with  hydrogen,  but  more  than  one 
with  oxygen. 

2.  The  second  argument  in  favor  of  the  hypothesis  of 
double  and  triple  linkage  is  that,  when  the  unsaturated 
compound  becomes  saturated,  an  equal  number  of  atoms 
or  groups  is  added  to  each  of  the  atoms  between  which  the 
complex  linkage  is  assumed.  This  indicates  that  there  is 
some  condition  between  these  atoms  that  affects  both  in  the 
same  way. 

It  is  held  by  some  that,  in  using  the  signs  indicating 
double  and  triple  linkage,  we  go  further  than  we  are  justi- 
fied in  going  by  what  we  actually  know,  This  is  undoubt- 
edly true.  We  do  not  know  that  anything  which  can 
fairly  be  called  double  linkage  ever  occurs  in  a  compound. 
At  the  same  time,  taking  all  the  facts  into  consideration, 
the  hypothesis  seems  justified,  but  the  sign  must  not  be 
interpreted  literally.  Whether  double  linkage  exists  in 
ethylene  or  not,  this  hydrocarbon  and  a  large  number  of 
related  compounds  have  a  certain  property  in  common 
which  may  conveniently  be  expressed  by  the  same  sign  in 
them  all. 

The  common  property  is  their  power  to  take  up  two 
atoms  of  hydrogen,  bromine,  etc.,  or  a  molecule  of  hydro- 
chloric, hydrobromic  acid,  etc.  The  double  line  =  be- 
tween carbon  atoms  may  then  be  used  as  a  sign  of  the 
ethylene  condition,  whatever  that  may  be,  and  it  indicates 
the  power  on  the  part  of  the  compound  represented  to 
take  up  two  additional  univalent  atoms.* 

Ethylene  and  Derivatives. — In  connection  with  ethane 
derivatives  it  was  stated  that  two  chlorides  are  known, 
both  of  which  have  the  formula  C2H4C12.  One  of  these 

*  For  an  instructive  discussion  of  this  subject  see  particularly 
Lessen,  Annalen  der  Chemie,  vol.  204,  p.  265. 


UNSATU&A.TED  COMPOUNDS.  233 

is  obtained  from  aldehyde  by  substituting  two  chlorine 
atoms  for  the  oxygen  atom;  hence  its  formula  was  assumed 
to  be  CHC12.CH3.  The  isomeric  compound  has  the  formula 
CH2C1 


H2C1 

The  latter  is  obtained  from  ethylene  by  direct  addition 
of  chlorine.  Hence  it  is  concluded  that  ethylene  itself  is 
symmetrical,  i.e.,  that  each  carbon  atom  in  it  holds  in 
combination  two  hydrogen  atoms  as  expressed  by  the  for- 
mula H2C=CH2. 

Propylene,  etc. — The  remaining  hydrocarbons  of  this 
series  are  obtained  for  the  most  part  by  treating  the  chlo- 
rides, bromides,  or  iodides  of  the  hydrocarbons  of  the 
methane  series  with  alcoholic  potash,  by  which  means  the 
molecule  C1H,  BrH,  or  IH  is  abstracted  from  the  com- 
pound. Thus,  from  C3H7 1  is  obtained  C3rI6;  from  C4H9I 
is  obtained  C4H8,  etc. 

In  many  cases  the  method  of  formation  of  the  hydro- 
carbon shows  at  once  what  its  constitution  is.  In  some 
cases  a  doubt  exists  even  after  all  the  methods  of  forma- 
tion and  the  products  of  decomposition  are  taken  into 
consideration. 

Alcohols. — Theoretically  a  series  of  alcohols  is  possible, 
derived  from  the  hydrocarbons  of  the  ethylene  series  by 
the  substitution  of  one  hydroxyl  group  for  one  hydrogen 
atom.  Only  one  such  alcohol  is  well  known.  This  is  allyl 
alcohol,  C3H5.OH,  or  CH2=CH.CH2.OH. 

Evidence. — Allyl  alcohol  differs  from  propyl  alcohol  in 

containing  two  hydrogen  atoms  less.    Now,  by  treating  allyl 

alcohol  with  nascent  hydrogen,  it  is  converted  into  normal 

propyl  alcohol,  which,  as  we  have  seen,  has  the  constitution 

H    H    H 

H — C— C — C — OH.     Hence  it  is  assumed   that  in  allyl 


alcohol,  as  well  as  in  propyl  alcohol,  the  hydroxyl  is  in 
combination  with  one  of  the  terminal  carbon  atoms,  and, 


234    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

accordingly,  it  must  be   either   CH2=CH — CH2.OH   or 
CH3— CH=CH.OH. 

Acrylic  acid,  which  is  obtained  from  allyl  alcohol  by 
oxidation,  is  formed  from  /3-iodo-propionic  acid  by  the  ab- 
straction of  hydriodic  acid.  /3-Iodo-propionic  acid,  further, 
is  formed  from  acrylic  acid  by  the  addition  of  hydriodic 
acid.  If  the  formula  for  /9-iodo-propionic  acid,  CH2I — 
CH2— COOH,  is  correct,  it  follows  that  the  formula 
CH2— CH — COOH  for  acrylic  acid  is  more  probable  than 
CH3— CH^COOH.  Indeed,  according  to  the  valency 
hypothesis,  the  latter  formula  represents  an  impossible 
compound.  If  the  formula  CH2=CH — COOH  for  acrylic 
acid  is  correct,  then  the  first  of  the  two  formulas  above 
given  for  allyl  alcohol  is  probably  correct. 

Adds. — Acrylic  acid  apparently  bears  the  same  relation 
to  allyl  alcohol  that  acetic  acid  bears  to  ordinary  alcohol. 
Still,  ordinary  oxidizing  agents  do  not  convert  the  alcohol 
into  the  acid. .  This  is  probably  due  to  the  instability  of 
the  unsaturated  compound.  Chromic  acid  converts  allyl 
alcohol  into  the  corresponding  aldehyde,  acrolein,  but  con- 
tinued action  of  the  oxidizing  agent  leads  to  the  formation 
of  formic  acid.  On  the  other  hand,  acrolein  is  converted 
into  acrylic  acid  by  means  of  a  less  active  agent  than 
chromic  acid,  as,  for  example,  silver  oxide.  Further,  if 
the  alcohol  is  first  converted  into  a  saturated  compound 
by  the  addition  of  bromine,  the  resulting  dibrompropyl 
alcohol  conducts  itself  towards  oxidizing  agents  the  same 
as  normal  propyl  alcohol.  It  yields  a  dibrompropionic 
acid,  and  this,  when  treated  with  zinc,  loses  bromine  and 
yields  acrylic  acid.  The  reactions  are : — 

CH2  CH2Br 

II  I 

1.  CH          +       Br2      =      CHBr     . 

CH2OH  CH2OH 

CH2Br  CH2Br 

I  I 

2.  CHBr  by  oxidation  yields  CHBr    . 

CH3OH  COOH 


VNSATURATED  COMPOUNDS.  235 

CH2Br  CH2 

3.      CHBr    +    Zn    =    CH     +     ZnBr2 . 

COOH  COOH 

CH2 

II 
These  changes  lead  to  the  formula  CH        for  acrylic 

COOH 

acid.  This  acid  is  the  first  of  a  series,  each  member  of 
which  differs  from  the  corresponding  member  of  the  series 
CnH2nO2  by  containing  two  hydrogen  atoms  less. 

Acetylene.  —  By  treating  symmetrical  dibromethane, 
CH2Br 

i,  with  an  alcoholic  solution  of  potassium  hydroxide, 
H2Br 

hydrobromic  acid  is  abstracted,  and  acetylene,  a  hydro- 
carbon of  the  formula  C2H2,  is  formed : — 

C2H4Br2  2HBr    —    C2H2  . 

This  compound  is  a  representative  of  a  class  of  unsatu- 
rated  compounds,  differing  from  the  ethylene  derivatives 
in  composition  the  same  as  the  latter  differ  from  the  marsh- 
gas  derivatives.  The  acetylene  compounds  have  the  power 
of  taking  up  four  atoms  of  chlorine  or  bromine  or  two  mole- 
cules of  hydrochloric  or  hydrobromic  acid.  The  reactions 
with  acetylene  are  illustrated  by  the  following  examples : — 

C2H2     +        4Br    =     C2H2Br,; 
C2H2     +    2HBr    =    C2H4Br2. 

In  regard  to  the  condition  existing  between  the  carbon 
atoms  in  acetylene  and  its  analogues  our  knowledge  is  in 
much  the  same  state  as  in  regard  to  the  condition  between 
the  carbon  atoms  in  ethylene.  The  arguments  advanced 
in  support  of  the  hypothesis  of  double  linkage  for  ethylene 
and  its  derivatives  may  be  used  in  modified  form  in  sup- 
port of  the  hypothesis  of  triple  linkage  for  acetylene. 
Whenever  saturation  of  a  compound  in  which  the  condi- 
tion of  triple  linkage  is  believed  to  exist  takes  place,  two 
atoms  or  groups  are  added  to  each  of  the  atoms  between 


236    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

which  the  triple  linkage  occurs.     This  is  illustrated  by  the 
following  reactions :  — 

H2C~CH2     +     4Br    =    Br2H2C— CH2Br2 ; 
H— C=N     +    4H     =    HSC— NH2. 

Just  as  the  sign  for  double  linkage  is  intended  mainly  to 
call  to  mind  the  fact  that  the  compound  in  which  it  occurs 
has  certain  properties  which  are  found  in  ethylene,  and 
may,  therefore,  be  called  the  sign  of  the  ethylene  condition; 
so  the  sign  for  triple  linkage  should  be  regarded  as  the  sign 
of  the  acetylene  condition,  and  when  it  is  found  in  a  for- 
mula it  indicates,  on  the  part  of  the  compound  represented, 
the  power  to  take  up  four  atoms  or  groups  and  thus  to  be- 
come saturated. 

Compounds  containing  a  Smaller  Proportion  of  Hydrogen. 
— There  are  several  hydrocarbons  known  which  are  in  some 
respects  apparently  analogous  to  those  already  considered, 
but  differing  from  them  in  containing  a  smaller  proportion 
of  hydrogen.  The  principal  of  these  are  valylene,  C5H6, 
and  dipropargyl,  C6H6.  These  take  up  bromine  in  larger 
proportion  than  the  hydrocarbons,  which  contain  more 
hydrogen.  Thus,  valylene  forms  the  compound  C5H6Br6, 
and  dipropargyl  forms  the  compound  C6H6Br8.  The  pro- 
ducts, it  will  be  noticed,  are  saturated,  the  former  being  a 
substitution-product  of  pentane,  C5H12,  and  the  latter  bearing 
a  similar  relation  to  hexane,  C6HU.  Not  enough  is  known 
about  these  substances  to  lead  to  a  definite  conclusion  in 
regard  to  their  structure.  It  seems  probable,  however,  that 
they  are  similar  to  ethylene  and  acetylene.  Valylene  may 
be  represented  by  the  formula  CH3 — C — C=CH,  and  dipro- 

CH2 

pargyl  by  CH~ C— CH2-CH2— C&CH.     The  formulas 
should,  however,  be  regarded  as  tentative. 


BENZENE  DERIVATIVES.  237 


CHAPTER  XVI. 

BENZENE   DERIVATIVES.      (AROMATIC    COMPOUNDS.) 

THERE  is  a  large  group  of  compounds,  the  members  of 
which  possess  the  property  in  common  that  the  hydro- 
carbon, benzene,  can  be  obtained  from  them.  Benzene 
itself  has  the  formula  C6H6.  Just  as  the  members  of  this 
group  of  compounds  yield  benzene  as  a  decomposition-pro- 
duct, so,  also,  they  may  be  built  up  from  benzene  by  the 
introduction  of  a  variety  of  groups  or  elements  in  the  place 
of  hydrogen.  All  these  compounds  bear  relations  to  ben- 
zene similar  to  those  which  the  fatty  compounds  bear  to 
marsh-gas.  In  studying  the  aromatic  compounds,  then,  it 
is  of  first  importance  to  determine  the  constitution  of  ben- 
zene itself,  as  the  constitution  of  the  derivatives  cannot  be 
understood  until  that  of  the  benzene  is  known. 

Constitution  of  Benzene. — Whatever  view  may  be  enter- 
tained regarding  the  structure  of  benzene,  the  following 
facts  must  be  accounted  for : — 

1.  Of  the  substitution-products  of  benzene  which  contain 
one  substituting  group,  only  one  variety  is  known. 

2.  Of  the  substitution-products  of  benzene  which  contain 
two  substituting  groups,  three  varieties  are  known,  and 
only  three. 

3.  Of  the  substitution-products  of  benzene  which  contain 
three  substituting   groups,  more  than  three  .varieties  are 
known,  except  in  case  the  substituting  groups  are  all  of  the 
same  kind,  in  which  case  only  three  varieties  are  known. 

4.  Six  and  only  six  atoms  of  bromine,  chlorine,  etc.,  can 
be  added  directly  to  benzene. 

A  great  deal  of  ingenious  experimenting  has  been  gone 
through  with  for  the  purpose  of  testing  the  first  of  these 
statements.  The  methed  adopted  may  be  briefly  described. 
Starting  with  benzene,  C6H6,  bromine  was  substituted  for 
one  atom  of  hydrogen,  a  product,  C6H5Br,  being  thus  formed. 
Now  some  other  element  or  group,  say  NO2,  was  substituted 


238    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

Br 
for  a  second  atom  of  hydrogen,  the  product  C6H4^ 

XNO2 

being  thus  obtained.  It  is  plain  that  the  group  NO2  must 
occupy  a  place  in  the  molecule  different  from  that  which 
the  bromine  occupies.  Now  hydrogen  was  substituted  for 
bromine,  leaving  the  compound  C6H5(NO2)  with  the  group 
NO2  presumably  occupying  the  same  place  that  it  did  in 

Br 
the  compound  C6H4(          .     Finally,  bromine  was  substi- 


tuted  for  the  group  NO2,  and  a  compound,  C6H5Br,  was 
obtained  in  which  the  bromine  occupied  a  different  place 
from  that  occupied  in  the  first  compound  of  the  same  com- 
position. The  two  compounds  were  found  to  be  identical, 
and  the  conclusion  is  drawn  that  two  of  the  hydrogen 
atoms  bear  the  same  relation  to  the  molecule.  In  a  similar 
way  the  examination  has  been  extended  to  all  six  of  the 
hydrogen  atoms,  and  the  result  reached  is  in  accordance 
with  the  first  general  statement.  It  seems,  therefore,  that 
all  six  hydrogen  atoms  in  benzene  bear  the  same  relation 
to  the  molecule.  From  this  it  follows  that  the  molecule 
of  benzene  is  symmetrical.  Each  one  of  the  hydrogen 
atoms  must  be  in  combination  with  a  single  carbon  atom, 
and  what  is  true  of  one  of  them  must  be  true  of  all  the 
others.  If  the  attempt  is  made  to  represent  these  ideas  by 
a  formula,  it  is  plain  that  the  formula  must  differ  in  some 
way  from  all  those  with  which  we  have  thus  far  had  to  deal. 
No  one  of  these,  representing  a  molecule  with  more  than  two 
carbon  atoms,  is  symmetrical.  They  represent  the  atoms 
as  arranged  in  chains  open  at  both  ends.  The  simplest 
way  in  which  the  symmetry  of  benzene  can  be  represented 
is  by  means  -of  a  circle.  We  may  suppose  the  six  atoms  of 
carbon  arranged  at  equal  distances  in  a  circle,  and  the  six 
hydrogen  atoms  in  combination  with  them,  thus:  — 


BENZENE  DERIVATIVES. 


239 


HC 


Of  course,  the  curved  line  has  no  special  significance, 
and  to  bring  this  formula  in  harmony  with  other  chemical 
formulas  we  may  write  it  thus : — 


HC 


CH 


This  formula,  then,  symbolizes  the  fact  that  each  of  the 
six  hydrogen  atoms  of  benzene  bears  the  same  relation  to 
the  molecule. 

Examining  the  formula  with  reference  to  the  derivation 
of  di-substitution-products,  we  are  led  to  the  conclusion 
that  the  compound  represented  by  it  ought  to  yield  three 
classes  of  di-substitution-products.  Numbering  the  hydro- 
gen atoms  thus : — 

1 

H 
C 


6HC 


5HC 


CH2 


CHS 


240    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

it  is  plain  that  there  are  three  kinds  of  relations  which  any 
one  hydrogen  atom  bears  to  the  others  considered  indi- 
vidually. Thus,  the  relation  of  1  to  2  is  different  from 
that  of  1  to  3  or  1  to  4.  The  relation  of  1  to  5  is  identical 
with  that  of  1  to  3,  and  that  of  1  to  6  is  identical  with  that 
of  1  to  2.  Hence,  if  X  be  substituted  for  H.I,  and  another 
X  for  another  hydrogen,  three  different  products  ought  to 
result  according  as  the  second  X  takes  the  place  of  H.2, 
H.3,  or  H.4.  The  same  statements  hold  good,  whether  we 
start  with  H.I  or  H.2,  or  any  of  the  other  hydrogen  atoms. 

It  will  thus  be  seen  that  the  formula  is  in  strict  harmony 
with  the  observed  fact  that  there  are  three  and  only  three 
classes  of  di-substitution-products  of  benzene. 

In  a  similar  way  it  can  be  shown  that  the  formula  is 
in  harmony  with  the  fact  that  of  the  tri-substitution-pro- 
ducts  containing  different  substituting  groups  there  are 
more  than  three  varieties,  while  of  those  in  which  all 
three  substituting  groups  are  the  same  there  are  only 
three  varieties;  though  not  much  weight  can  be  attached 
to  this  as  yet,  as  the  subject  has  not  been  investigated  to 
a  sufficient  extent  to  furnish  a  sufficient  basis  of  facts. 

There  is  another  fact  which,  if  interpreted  by  the  aid 
of  the  valency  hypothesis,  also  speaks  in  favor  of  the 
above  formula.  This  formula  does  not  account  for  all 
the  bonds  of  carbon.  It  is  not  known  what  relation 
exists  between  these  carbon  atoms.  Various  formulas 
have  been  suggested  with  the  object  of  showing  how  the 
bonds  are  disposed  of,  but  they  are  all  open  to  the  serious 
objection  that  they  express  relations  about  which  we  know 
practically  nothing.  At  the  present  stage  of  our  knowledge 
it  makes  little  difference  whether  we  write  the  formula : — 


or 
CH  HC 


both  of  which  are  in  use.     It  is  better  not  to  write  either, 
but  to  use  the  simple  figure  above  given,  which  does  not 


BENZENE  DERIVATIVES.  241 

attempt  to  tell  the  entire  story,  but  simply  to  express  by 
a  symbol  certain  ideas  to  which  we  are  led  by  a  considera- 
tion of  the  facts  known  to  us.  Using,  then,  the  simplest 
formula, 


HC 

C 
H 

though  for  the  argument  it  is  immaterial  which  one  of  the 
three  we  use,  the  idea  suggests  itself  that  this  hydrocarbon 
ought  to  be  able  to  take  up  bromine ;  it  appears  to  be  un- 
saturated.  If  each  carbon  atom  has  the  power  to  hold 
four  atoms,  then  plainly  each  one  ought  to  be  able  to  take 
up  one  atom  of  bromine.  By  treatment  with  bromine, 
benzene  ought  to  yield  an  addition-product  C6H6Br6,  or 

HBr 
C 

BrHC  ^  ^  CHBr 

BrHC  L          J  CHBr 


In  accordance  with  this  is  the  fact  that,  when  benzene 
is  treated  with  bromine  in  the  sunlight,  a  product,  C6H6Br6, 
is  actually  formed,  and  not  C6H6Br8,  as  in  the  case  of  the 
isomeric  hydrocarbon  dipropargyl. 

Taking  all  the  facts  together,  then,  it  is  clear  that  the 
formula  given  for  benzene  represents  to  the  properly 
trained  mind  some  facts  of  fundamental  importance.  No 
one  claims  that  this  formula,  any  more  than  any  other  in 
use,  represents  the  actual  arrangement  of  the  atoms  in 
space,  or  that  the  formula  has  anything  whatever  to  do 
with  facts  of  this  order.  But  it  undoubtedly  does  repre- 
sent certain  important  truths. 


242    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

Although  our  present  knowledge  will  not  permit  us  to 
express  with  any  confidence  more  in  regard  to  the  struc- 
ture of  benzene  than  is  expressed  by  the  simple  formula, 
still  the  question  as  to  the  way  in  which  the  carbon  atoms 
are  united  in  benzene  is  one  that  may  legitimately  be 
made  the  subject  of  investigation,  and  it  has  received  a 

H 

C 

^, 

HC 
great  deal  of  attention.     The  formula 

HC 


represents  a  compound  in  which  the  ethylene  condition 
occurs  three  times.  So  far  as  the  formation  of  addition- 
products  is  concerned,  this  formula  is  satisfactory.  On  the 
other  hand,  it  does  not  represent  a  perfectly  symmetrical 
compound,  and  the  first  fact  in  regard  to  benzene  which 
must  be  represented  is  symmetry.  Considering  any  carbon 
atom  represented  in  this  formula,  it  is  plain  that  on  one  side 
it  is  joined  to  a  carbon  atom  by  single  linkage,  and,  on  the 
other,  it  is  joined  to  a  carbon  atom  by  double  linkage. 
Apparently  two  di-substitution-products  of  the  formulas  :— 

X  X 

C  C 

CH  HC 

and 

CH  HC 


are  possible  in  such  a  compound.  But  experiment  has 
shown  that  the  two  compounds  with  the  substituting 
groups  in  the  relations  represented  in  these  formulas  are 
identical.*  In  view  of  the  facts  several  efforts  have  been 

*  According  to  recent  experiments,  there  appear  to  be  two  ortho- 
phthalic  acids.  If  this  should  be  verified,  the  fact  would  furnish 
strong  evidence  in  favor  of  the  unsymmetrical  formula  of  benzene- 


BENZENE  DERIVATIVES. 


243 


made  to  represent  the  constitution  of  benzene  by  a  per- 
fectly symmetrical  formula  which  should  at  the  same  time 
be  in  accordance  with  the  general  notions  regarding  struc- 
ture. It  has  been  suggested  that  the  constituents  of  ben- 
zene are  arranged  in  the  form  of  a  regular  prism,  thus, 

CH 


HC 


If  this  is  examined  simply  with  reference  to  the  linkage 
between  the  carbon  atoms,  it  is  identical  with  this  formula, 

CH 


HC 
HC 


CH 


In  this  modified  prism  formula  there  is  no  double  linkage 
between  carbon  atoms.  The  facts  above  referred  to  speak 
in  favor  of  the  prism  formula,  but  they  do  not  decide  the 
question.  Recently  a  chemical  method  of  dealing  with 
the  problem  has  been  suggested  which  may  throw  light 
upon  it.  The  considerations  upon  which  the  method  is 
based  are  these :  When  a  compound  in  which  double  link- 
age occurs  becomes  saturated,  the  groups  or  atoms  which 
are  taken  up  are  added  to  those  carbon  atoms  between 
which  the  double  linkage  occurs.  If,  therefore,  it  were  pos- 
sible to  add  two  atoms  or  groups  to  benzene,  and  it  could 
be  shown  that  they  are  united  with  carbon  atoms  in  the 

HX 
C 


position  represented  in  this  formula 


XHC 


244    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 


such  a  fact  would  be  evidence  in  favor  of  the  view  that 
double  linkage  previously  existed  between  the  two  adjoining 
carbon  atoms.  If  the  other  formula  were  correct,  the  posi- 
tion of  two  groups  or  atoms  in  a  di-addition-product  would 

H 
C 


be  represented  by  this  formula 


XHC 


or   this 


As  there  are  ways  of  deter- 


mining which  of  these  positions  atoms  or  groups  occupy 
in  benzene,  this  method  would  appear  to  be  promising.  It 
has,  however,  been  tested  and  found  to  be  entirely  unsatis- 
factory. Von  Baeyer,  who,  of  late  years,  has  given  most 
attention  to  the  question  of  the  constitution  of  benzene, 
has  reached  the  conclusion  that  the  formula  first  proposed 
by  Claus  is  the  one  most  in  accordance  with  the  facts. 
This  is— 


He  concludes,  further,  that,  when  addition  takes  place,  the 
distinctive  benzene  condition  is  destroyed. 

Besides  the  chemical  methods  above  referred  to,  certain 
physical  methods  have  been  applied  to  the  problem  of 
determining  the  constitution  of  benzene.  One  of  them 


BENZENE  DERIVATIVES.  245 

is  based  upon  the  determination  of  the  heat  evolved  in 
the  complete  combustion  of  benzene ;  and  another  upon 
observations  on  the  refracting  power  of  benzene.  As  the 
former  method  leads  to  the  conclusion  that  there  are  nine 
single  linkages  in  benzene;  and  the  latter  method  leads 
to  the  conclusion  that  the  formula  with  three  double  link- 
ages and  three  single  linkages  is  correct,  it  is  clear  that 
one  of  the  methods  must  be  defective,  and  further  investi- 
gation is  necessary. 

Substitution-products  of  Benzene. — Of  mono-substitution- 
products  there  is  only  one  variety.  There  is  only  one 
monochlorobenzene,  C6H5C1 ;  only  one  hydroxybenzene,  or 
phenol,  C6H5OH;  only  one  benzoic  acid,  C6H5.COOH; 
only  one  toluene,  C6H5.CH3,  etc.,  etc.  The  constitution 
of  most  of  these  derivatives  is  very  simple.  There  is  a 
peculiarity,  however,  connected  with  those  which  are 
formed  by  the  introduction  of  a  hydrocarbon  residue  in 
the  place  of  one  hydrogen  atom  of  benzene.  The  simplest 
compound  formed  in  this  way  is  toluene,  which  consists  of 
beozene  in  which  the  methane  residue,  methyl,  CH3,  takes 
the  place  of  a  hydrogen  atom ;  if,  instead  of  the  residue  CH3, 
ethyl,  C2H5,  is  introduced,  ethylbenzene,  C6H5.C2H5,  which 
is  plainly  a  homologue  of  toluene,  is  obtained  ;  so,  also,  the 
residues  C3H7,  C4H9,  C5HU,  etc.,  may  be  introduced,  and 
thus  an  homologous  series  of  aromatic  hydrocarbons  is 
obtained,  all  of  which  are  mono-substitution-products  of 
benzene.  These  may,  further,  all  be  regarded  as  substi- 
tution-products of  the  hydrocarbons  of  the  methane  series. 
Accordingly,  of  toluene  and  ethylbenzene,  which  are  mono- 
substitution-products  of  methane  and  ethane  respectively, 
only  one  variety  is  possible;  while  of  the  next  homologue, 
or  propylbenzene,  C6H5.C3H7,  two  varieties  are  possible, 
corresponding  to  the  a-  and  /5-mono-substitution-products 
of  propyl,  or  to  the  propyl  and  isopropyl  compounds 
(which  see).  The  principal  members  of  the  series  of 
hydrocarbons  thus  referred  to  are : — 

Benzene,  C6H6. 

Toluene  or  methylbenzene,     C7H8   or  C6H5.CH3. 

Ethylbenzene,  C8H10  or  C6H5.C2H5. 

Propylbenzene,  C9H12  or  C6H5.C3H7. 

Butylbenzene,  C10HU  or  C6H5.C4H9. 

Amylbenzene,  CUH16  or  C6H5.C5HU. 


246    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

Of  these  hydrocarbons  two  classes  of  mono-substitution- 
products  are  possible,  viz.,  those  in  which  the  substituting 
group  or  element  is  situated  in  the  benzene  ring,  and  those 
in  which  the  substituting  group  or  element  is  situated  in 
the  other  residue.  These  other  residues,  however  they 
may  be  constituted,  are  known  as  side  chains.  It  is  plain 
that  substitution-products  of  the  latter  kind  correspond 
closely  to  those  of  the  hydrocarbons  of  the  methane  series, 
and  hence  they  need  no  special  treatment  here.  If  a  sub- 
stituting group  or  element  enters  into  the  benzene  ring  of 
any  of  these  hydrocarbons,  of  course  we  have  no  longer  to 
deal  with  mono-substitution-products  of  benzene. 

Di-substitution-products. — The  three  classes  of  di-deriva- 
tives  of  benzene,  which  we  have  above  recognized  as  pos 
sible,  have  been  designated  respectively  as  ortho,  meta,  and 
para  compounds,  or,  by  others,  as  1.2,  1.3,  and  1.4  com- 
pounds. The  former  expressions  are  to  be  preferred,  for 
they  are  independent  of  any  hypothesis  concerning  the 
positions  of  the  substituting  groups  It  is  usual  to  consider 
the  expressions  ortho  and  1.2,  meta  and  1.3,  para  and  1.4, 
as  identical,  but  this  implies  that  the  following  formulas 
have  been  proved,  while  they  have  not  been,  although  they 
have  been  rendered  extremely  probable : — 

XXX 

c  c  c 


HC 


N 


C  C  C 

H  H  X 

Ortho  compound.  Meta  compound.  Para  compound. 

What  we  really  know  is  that  there  are  three  classes  of 
these  di-substitution  products,  and  that  the  members  of 
any  one  of  these  classes  can  be  converted  into  one  another, 
thus  showing  that  they  are  allied.  Any  three  varieties  of 
a  di-substitution-product  of  benzene  may  be  taken  as  the 
basis  of  classification  of  all  the  di-substitution-products. 
Thus,  the  three  isomeric,  dicarbonic  acids  of  benzene, 


BENZENE  DERIVATIVES.  247 

.COOH 
C.H4^  ,  viz.,  phthalic,  isophthalic,  and  terephthalic 

^COOH 

acids  may  be  taken  for  this  purpose.  All  di-substitution- 
products  that  can  be  converted  into  phthalic  acid  are 
known  as  ortho  compounds ;  all  that  can  be  converted  into 
isophthalic  acid  are  known  as  meta  compounds ;  and  all 
that  can  be  converted  into  terephthalic  acid  are  known  as 
para  compounds. 

The  conversion  into  these  acids  need  not  be  direct.  If 
it  is  possible  to  convert  a  compound  into  another,  which, 
in  its  turn,  can  be  converted  into  one  of  the  above  acids, 
the  same  conclusion  is  drawn  as  in  the  case  of  a  direct 
conversion.  Of  course,  the  accuracy  of  the  conclusions 
drawn  with  reference  to  the  constitution  of  di  substitution- 
products  depends  upon  the  trustworthiness  of  the  reactions 
employed  in  effecting  the  conversions.  Some  reactions  em- 
ployed for  this  purpose  have  been  found  to  give  inaccurate 
results,  that  is  to  say,  the  products  resulting  from  an  appli- 
cation of  these  reactions  belong  to  different  series  from 
those  to  which  the  original  compounds  belonged.  It  is 
probable  that  some  compounds  now  classified  with  one  series, 
in  consequence  of  some  transformations,  may  be  found  by 
future  investigations  to  belong  to  a  different  series. 

The  formulas  given  above  as  representing  the  relative 
positions  of  the  substituting  groups  in  ortho-,  meta-,  and 
para- compounds  are  based  upon  the  following  facts: — 

It  will  be  shown  that  naphthalene  (which  see)  probably 
has  the  formula  : — 


By  oxidation  naphthalene  yields  phthalic  acid.    It  seems 
probable,  therefore,  that  the  carboxyl  groups  in  the  acid 


248    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 


have  the  same  relative  position  as  that  of  the  groups  num- 
bered 1  and  2  in  this  formula.  Consequently,  ortho-com- 
pounds, or  those  which  can  be  converted  into  phthalic  acid, 
are  assumed  to  have  their  substituting  groups  in  the  posi- 
tions marked  1.2  in  the  benzene;  or,  what  is  the  same 
thing,  the  substituting  groups  in  ortho-compounds  are 
combined  with  carbon  atoms  which  are  adjacent  in  the 
formula. 

It  will  also  be  shown  that  mesitylene  (which  see)  prob- 
ably has  the  formula  :  — 

CH3 


CH3— 


By  partially  oxidizing  this  hydrocarbon  one  methyl  is 
changed  to  carboxyl,  and  an  acid  is  obtained  of  the  for- 
mula : — 

COOH 


A 


HC 


CH3—  C 


CH 


C—  CH3 


C 
H 


When  this  acid  is  heated  with  lime,  carbon  dioxide  is 
given  off,  and  a  hydrocarbon  is  o'btained  of  the  formula  :  — 


BENZENE  DERIVATIVES. 


249 


H 
C 


HC 
CH3— C 


C-CH3 


C 
H 


Lastly,  when  this  hydrocarbon  is  oxidized,  both  the 
groups  CH3  are  converted  into  carboxyl,  COOH,  and  the 
product  is  isophthalic  acid.  Hence,  if  the  formula  of 
mesitylene  is  correct,  that  of  isophthalic  acid,  which  repre- 

H 
C 


sents  it  thus, 


HOOC 


CH 


,  is  also  correct. 


COOH 


CH 


By  exclusion,  terephthalic  acid  becomes  a  1.4  compound, 
and,  consequently,  all  para-compounds  are  1.4  compounds. 

Another  method  of  proof  is  founded  upon  the  determina- 
tion of  the  number  of  isomeric  substitition-products  that 
can  be  obtained  from  certain  di-derivatives  of  benzene. 
Take  the  three  xylenes,  for  example  : — 

II. 
CH3 


c—  CH3    HC 


HC 


HC 


L  Jc— CH3 


There  are  three  hydrocarbons  that   are  known  to  be 
dimethyl  derivatives  of  benzene.     The  benzene  hypothesis 


250    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 


furnishes  three  formulas,  but  we  cannot  determine  by  in- 
tuition which  one  of  the  formulas  to  give  to  one  particular 
hydrocarbon,  and  yet,  if  the  benzene  hypothesis  is  of  value, 
only  one  of  the  three  formulas  is  assignable  to  any  par- 
ticular hydrocarbon. 

An  examination  Of  formula  III.  will  show  that  each  of 
the  four  hydrogen  atoms  belonging  to  the  benzene  bears 
exactly  the  same  relation  to  the  molecule.  Interpreting 
this  formula  in  the  simplest  way,  we  are  led  to  the  conclu- 
sion that  the  compound  which  it  represents  ought  to  yield 
but  one  mono-substitution-product  with  any  one  agent. 
This  is  not  true  of  the  other  two  formulas  I.  and  II.  The 
compound  represented  by  formula  I.  ought  to  yield  two 
different  mono-substitution  products  with  any  one  reagent, 
and  the  compound  represented  by  formula  II.  ought  to 
yield  three,  thus : 

Form.  I. 


HC 


C— CH. 


HC 


HC 


C— CH3 


CH 


CH3 
C 


HC 


11 


C 
II 


cx 


HC 


C— CH3    HC 


CH 


HC 


C-CH3  X 


CH3 
C 


CH 


C— CH, 


It  has  been  shown  that  from  one  of  the  three  xylenes 
only  one  variety  of  mono  substitution-products  can  be  ob- 
tained, and  the  conclusion  is  drawn  that  to  this  one,  for- 
mula III.  should  be  assigned.  This  is  paraxylene,  so  that 


BENZENE  DERIVATIVES.  251 

we  are  led  by  this  method  of  reasoning  to  exactly  the  same 
conclusion  as  by  the  methods  already  considered.  This 
method  has  not  been  fully  applied  in  the  case  of  the  two 
other  hydrocarbons,  but  sufficient  is  known  in  connection 
with  other  di-substitution-products  of  benzene  to  show  that 
some  of  them  yield  two,  and  only  two,  kinds  of  derivatives 
by  the  introduction  of  one  more  substituting  group,  while 
others  yield  three.  The  former  are  the  ortho-,  the  latter 
the  meta-compounds. 

The  above  will  give  a  fair  idea  of  the  basis  upon  which 
the  expressions  1.2,  1.3,  and  1.4  rest.  Some  of  the  prin- 
cipal di-substitution-products  of  benzene  are  given  in  the 
following  table,  which  shows  also  to  which  series  the  com- 
pounds, belong: 

Ortho.  Meta.  Para. 

Phthalic  acid,  Isophthalic  acid,  Terephthalic  acid, 

Orthoxylene,  Isoxylene,  Paraxylene, 

Salicylic  acid,  Oxybenzoic  acid,  Paroxybenzoic  acid, 

Pyrocatechol,  Kesorcinol,  Hydroquinol, 

Orlhodinitrobenzene,  Metadinitrobenzene,  Paradinitrobenzene, 

Orthodibrombenzene,  Metadibrombenzene,  Paradibrombenzene. 

Tri-substitution-products. — One  of  the  most  important 
of  the  tri-substitution  products  of  benzene  is  mesitylene. 
The  formula  of  this  hydrocarbon  is  C9H12.  By  oxidation 
it  yields,  according  to  the  extent  to  which  the  action  is 
allowed  to  proceed,  three  different  products.  The  first, 
rnesitylenic  acid,  C8H9.COOH,  is  monobasic;  the  second, 
uvitic  acid,  C7H6,(COOH)2,  is  dibasic;  and  the  third,  tri- 
mesitic  acid,  C6H3.(COO)3H,  is  tribasic.  All  of  these 
acids,  when  heated  with  lime,  yield  either  benzene  itself  or 
derivatives  of  benzene.  Hence,  it  is  concluded  that  mesi- 
tylene is  benzene  in  which  three  methyl  groups  have  taken 
the  place  of  three  hydrogen  atoms,  as  expressed  in  the  for- 
mula C6H3(CH3)3.  By  oxidation  each  one  of  these  groups 
in  turn  is  converted  into  carboxyl,  yielding  thus  the  three 
acids  above  mentioned.  It  still  remains,  however,  to  decide 
what  positions  these  three  substituting  groups  in  benzene 
occupy. 

The  following  considerations  lead  to  the  formula  for 
mesitylene  given  on  page  248. 

When  acetone  is  treated  with  concentrated  sulphuric 
acid  water  is  abstracted,  and  the  residues  of  three  mole- 
cules unite  to  form  mesitylene.  It  seems  fair  to  assume 


252    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 


that  the  three  residues  have  the  same  constitution,  as  they 
are  formed  under  exactly  the  same  conditions,  from  the 
same  compound.  If  they  are  the  same,  each  must  be 
C3H4.  Three  such  residues  might  be  formed  from  acetone, 
thus : — 

Acetone  is  CH3 — CO — CH3.     Three  molecules  may  be 
arranged : — 

CH3 


H 
CH 

H 

0|C 

"CH 

|o 

HJ 

1 

-C 

o 
5 

\ 

C-CH3 
/ 

If  water  is  abstracted  in  the  manner  indicated  by  the 
lines,  there  are  left  three  residues,  C3H4,  and,  if  these 
unite,  they  will  form  a  compound  of  the  constitution  rep- 
resented by  the  following  formula : — 

CH3 


HC 


CH3— C 


C— CH3 


This  is  the  formula  accepted  for  mesitylene ;  and  from 
this  the  conclusion  is  drawn  that  meta- compounds  have 
their  substituting  groups  in  the  positions  1.3. 

If  this  formula  is  examined,  it  will  be  seen  that  each 
one  of  the  three  hydrogen  atoms  remaining  in  the  ben- 
zene occupies  a  position  like  that  occupied  by  the  other 
two.  Accordingly,  if  this  formula  is  correct,  we  should 
expect  to  find  that,  by  the  introduction  of  one  substituting 
group  into  mesitylene,  only  one  product  would  be  formed. 
This  has  been  found  to  be  true. 


BENZENE  DERIVATIVES.  253 

Besides  mesitylene  there  are  many  tri-substitution-pro- 
ducts  of  benzene  known,  containing  such  elements  as 
chlorine,  bromine,  and  iodine,  and  such  groups  as  the 
nitro-group,  NO2,  the  amido-group,  NH2,  the  sulphonic 
acid  group,  SO2OH,  etc.  The  method  by  which  the  posi- 
tion of  the  substituting  groups  in  these  compounds  is  de- 
termined is  this:  One  of  the  groups  is  split  off,  and  the 
constitution  of  the  resulting  di-substitution  product  is  de- 
termined as  above;  then,  from  the  original  compound 
some  other  group  is  split  off,  and  the  constitution  of  the  di- 
substitution- product  resulting  in  this  case  also  determined. 
It  is  thus  possible  to  judge  of  the  positions  of  the  three 
groups  with  reference  to  one  another.  There  are  not  many 
compounds,  however,  that  can  be  subjected  to  this  kind  of 
examination  with  satisfactory  results,  so  that  the  constitu- 
tion of  the  tri-derivatives  is  not  so  well  established  as  that 
of  the  di-derivatives. 

Peculiar  Benzene  Derivatives. — Among  benzene  deriva- 
tives there  are  three  classes  which  are  not  represented,  or 
not  so  well  represented,  among  the  fatty  compounds,  and 
hence  they  require  some  attention  here.  These  are  the 
phenols,  quinones,  and  azo-compounds.  • 

Phenols. — Phenols  are  the  hydroxyl  derivatives  of  ben- 
zene and  its  homologues,  formed  by  the  introduction  of 
hydroxyl  in  the  place  of  hydrogen  in  benzene.  The  cor- 
responding compounds  of  the  hydrocarbons  of  the  methane 
series  are  all  alcohols,  either  primary,  secondary,  or  ter- 
tiary. The  phenols  are,  however,  not  alcohols  in  the  sense 
in  which  that  term  has  been  used  up  to  the  present.  By 
oxidation  they  do  not  conduct  themselves  like  alcohols. 
If,  however,  by  the  expression  tertiary  alcohol  is  meant 
any  compound  which  contains  the  grouping  C(OH),  then 
the  phenols  are  all  tertiary  alcohols.  It  is,  perhaps,  better 
to  restrict  the  use  of  the  name  alcohol  to  the  hydroxyl 
derivatives  of  the  marsh-gas  hydrocarbons. 

The  presence  of  hydroxyl  in  phenols  can  be  shown  in 
the  same  way  that  it  was  shown  for  other  compounds 
containing  hydroxyl. 

There  are  monacid  phenols,  containing  only  one  hy- 
droxyl ;  diacid  phenols,  containing  two  hydroxyls ;  triacid 
phenols,  containing  three  hydroxyls,  etc. 

12 


254     PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

Quinones. — The  quinones  are  derived  from  benzene  and 
its  hoinologues  by  the  introduction  of  two  atoms  of  oxygen 
in  the  place  of  two  hydrogen  atoms  in  benzene.  Thus,  the 
simplest  quinone  has  the  formula  C6H4O2.  Whether  the 
two  oxygen  atoms  form  a  bivalent  group,  — O — O — ,  by 
combining  with  each  other  by  means  of  one  of  their  affini- 
ties each,  or  are  in  combination  with  carbon  in  the  car- 
bonyl  condition  C=O,  it  has  thus  far  been  impossible  to 
decide.  The  quinones  are  derived  from  para-compounds 
by  oxidation,  as  from  hydroquinone,  and  hence  it  is  con- 
cluded that  the  oxygen  atoms  in  the  quinones  occupy  the 
para-position  with  reference  to  each  other.  Accordingly, 
if  the  para- position  is  1.4,  the  formula  of  ordinary  quinone 
is  either 


A&o-  and  Diazo-compounds. — These  compounds,  as  their 
names  imply,  are  nitrogen  derivatives.  They  are  derived 
from  benzene  and  its  homologues  by  the  substitution  of 
nitrogen  for  hydrogen.  We  need  only  consider  those 
which  are  derived  from  benzene,  as  the  others  are  very 
closely  related  to  them.  The  diazo- derivatives  of  benzene 
are  obtained  from  the  salts  of  aniline  or  amidobenzene, 
C6H5.NH2,  by  the  action  of  nitrous  acid.  Thus,  aniline 
nitrate,  C6H5.NH2.HNO3,  yields  diazobenzene  nitrate; 
aniline  sulphate,  (C6H5.NH2).H2SO4,  yields  diazobenzene 
sulphate,  etc. 

If  we  consider  simply  the  empirical  formulas  of  the  salts 
of  diazobenzene  thus  obtained,  we  find  that  they  differ 
from  the  aniline  salts  in  containing  CfiH4N2  in  the  place  of 
C6H5NH2.  The  salts  consist  of  the  acids  plus  this  group. 
Thus,  the  nitrate  is  C6H4N2.HNO3 ;  the  sulphate  is 
C6H4N2  H2S04,  etc.  These  formulas  do  not,  however,  rep- 


BENZENE  DERIVATIVES. 


255 


resent  the  constitution  of  the  salts.  If  the  group  C6H4N2 
actually  existed  in  these  diazo  compounds,  it  is  plain  that 
they  would  be  di  substitution-products,  that  is  to  say,  two 
nitrogen  atoms  would  take  the  place  of  two  hydrogen  atoms 
of  benzene.  It  was  at  first  supposed  that  each  of  these 
nitrogen  atoms  played  the  part  of  a  univalent  element,  and 
the  diazo-compounds  were  looked  upon  as  analogous  to 
dichlorbenzene,  dinitrobenzene,  etc.,  thus : — 
N  Cl 

c  c 


HC 


CH 


c 

H 

Dichlorbenzene. 

It  was  soon  found,  however,  that  when  the  diazo-com- 
pounds  of  benzene  are  decomposed  they  generally  yield 
derivatives  of  benzene  in  which  the  group  C6H5  is  un- 
doubtedly present.  Thus,  the  following  decompositions  of 
diazobenzene  sulphate  yield,  in  each  case,  a  derivative 
containing  C6H. : — 

When  boiled  with  alcohol  the  products  are  ethyl-phenyl 
ether,  C6H5.O.C2H5,  nitrogen  and  sulphuric  acid : — 


C6H5.N2.HS04 
C2H50    H 


yield 


C6H5 
C2H50 


HSO4 
H 


When  boiled  with  water  the  products  are  phenol,  nitro- 
gen, and  sulphuric  acid  : — 


C6H5.N2.HS04 
OH    H 


yield 


OH 


HSO4 
H 


When  treated  with  hydriodic  acid  the  products  are  iodo- 
benzene,  nitrogen,  and  sulphuric  acid  : — 


C6H5.N2.HS04 
I       H 


yield 


C6H5 

I 


N. 


HSO4 
H 


256    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

Other  reactions  indicate  as  well  that  the  group  C6H5  is 
present  in  the  derivatives  of  diazobenzene.  But,  if  this 
group  is  present,  the  two  nitrogen  atoms  must  be  so  com- 
bined that  they  can  take  the  place  of  one  hydrogen  atom. 
Now,  if  two  nitrogen  atoms,  which  have  the  same  valency, 
be  combined,  they  must  either  form  a  neutral  group  with 
all  its  bonds  satisfied,  or  a  group  which  is  at  least  bivalent. 
Such  a  bivalent  group  will  be  formed,  for  instance,  if  two 
nitrogen  atoms  are  united  by  means  of  two  bonds  each, 
thus,  — N— N — .  If  this  group  should  be  substituted  for 
one  hydrogen  atom  of  benzene,  the  constitution  of  the  re- 
sulting compound  would  be  C6H5 — N=N — .  Such  a  com- 
pound would  be  unsaturated.  No  compound  of  the  formula 
C6H5N2  has  been  obtained,  but  all  the  derivatives  of  diazo- 
benzene can  be  explained  on  the  supposition  that  they  are 
derived  from  the  compound  C6H5 — N=N — . 

That  the  two  nitrogen  atoms  do  not  take  the  place  of 
two  hydrogen  atoms  but  only  of  one  is  shown,  further,  very 
clearly  by  the  fact  that  the  compound,  C6Br4(SO3H)(NH2), 
can  be  converted  into  a  diazocompound. 

Accepting  the  group  C6H5 — N=N —  as  the  foundation 
of  the  diazo-compounds,  these  may  be  represented  as  fol- 
lows : — 

C6H5 — N=N — Br,  diazobenzene  bromide, 

C6H5 — N=N — NO3,  diazobenzene  nitrate, 

C6H5 — N— N — HSO4,  diazobenzene  sulphate, 

C6H5 — N--N — OK,  diazobenzene  potassium, 

C6H5— N^N— NH(C6H5),  diazo-amidobenzene. 

Azobenzene  is  formed  by  the  reduction  of  nitrobenzene. 
Its  formula  is  C12H10N2.  As  nitrobenzene  contains  the 
group  C6H5  combined  with  nitrogen,  we  may  assume  that 
azobenzene  consists  of  two  such  groups  C6H5 — N=.  If 
these  combine  in  the  simplest  manner,  we  should  have  the 

C6H5-N 
formula  ||   ,  expressing  the  constitution  of  azoben- 

C6H6— N 
zene.     This  is  the  formula  now  generally  accepted. 

According  to  this,  the  azo-compounds  are  very  closely 
related  to  the  diazo-compounds.  Both  contain  the  group 
— N^N —  in  combination  with  C6H5.  In  reality,  the  azo- 
compounds  differ  very  much  in  their  chemical  conduct 
from  the  diazo-compounds.  The  decompositions  which  they 


BESZENE  DERIVATIVES.  257 

undergo  take  place  in  a  manner  entirely  different  from 
that  already  noticed  as  characterizing  the  decomposition  of 
diazo-compounds. 

This  difference  has  led  some  chemists  to  abandon  the 
formulas  above  given  for  the  diazo-compounds,  and  to  pro- 
pose others  in  their  place.  The  compounds  are  supposed 
to  be  ammonium  compounds  of  the  general  formula 
R— N— R' 

HI        .     They   contain   one   quinquivalent  and  one 
N 

trivalent  nitrogen  atom.  According  to  this  view,  the  re- 
lation between  aniline  nitrate  and  diazobenzene  nitrate  is 
shown  thus : — 

CTT  r\  TT 

e^s  ^6^5 

N— 0-NO2 ;  N— O— NO2 . 

Ill  III 

H3  N 

Aniline  nitrate.  Diazobenzene  nitrate. 

Against  the  latter  view  the  following  facts  speak  :  When 
diazobenzene  nitrate  is  treated  with  neutral  potassium 
sulphate  a  salt  of  the  formula  C6H5 — N2 — SO3K  is  formed 
according  to  this  equation  : — 

.  C6H—  N  —  N03+  S03KK=C6H—  N  — SO3K+NO3K. 

When  the  salt  thus  obtained  is  reduced  it  yields  a 
phenylhydrazine  derivative : — 

C6H6-N2-S03K-fH2-C6H-N2H2-S03K ; 

and,  finally,  when  this  product  is  treated  vvith  hydrochloric 
acid  the  hydrochloride  of  phenylhydrazine  is  formed : — 

CJL— N2H2— SO3K+HC1=: 

QH—  N2H—  HC1+HKSO,. 

But  the  constitution  of  phenylhydrazine  is  represented 
by  the  formula  C6H5 — HN — NH2.  Its  formula  is  easily 
understood  if  the  constitution  of  diazobenzene  nitrate  is 
CgH5 — N— N — NO3.  In  this  case  the  change  from  the 
nitrate  to  phenylhydrazine  consists  simply  in  the  substitu- 
tion of  hydrogen  for  the  NO3  group  and  the  saturation  of 
the  compound  by  hydrogen.  In  the  other  case  the  succes- 
sive products  must  be  represented  in  this  way ; — 


258  PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

C6H5— N— NO3  C6H5— N— SO3K  C6H5— NH—  SO3K  C6H6— 

1        '         1         '        JH         ' 

The  formula  to  which  we  are  thus  led  for  phenylhydrazine 
is  not  correct. 

It  must  be  acknowledged  that  this  reasoning  is  not  satis- 
factory, for,  even  though  a  compound  of  the  last  formula 
given  was  formed,  this  might  easily  pass  over  into  the  more 
stable  form  of  phenylhydrazine, 

A  better  piece  of  evidence  in  favor  of  the  formula 
CfiH. — N=N — ,  and,  therefore,  against  the  formula 
C6H— N— 

||!  ,  is  that  furnished  by  the  compounds  formed  by 
N 

the  action  of  diazo-compounds  or  amido  derivatives  of 
benzene.  Thus,  for  example,  when  diazo-benzene  acts 
upon  dimethyl-aniline,  a  compound  of  the  formula, 
C6H5— N=N— C6H4— N(CH3)2,  is  formed.  When  this  is 
reduced  it  yields  aniline  and  amido-dimethyl-aniline, 
NH2.C6H4.N(CH3)2:— 

C6H5— N=N— C6H4— 'N(CH3)2 +4H=C6H6-NH2 +NH2.C6H4.N(CH3)2. 

As  will  be  seen  it  would  be  very  difficult  to  explain 
these  facts  if  the  diazo-compounds  contained  the  group, 

— N— 


I  ' 


Azoxy-  and  Hydrazo-compounds. — Among  other  nitrogen 
compounds  which  are  related  to  the  diazo-  and  azo-com- 
pounds  are  the  azoxy-  and  hydrazo-compounds.  These  as 
well  as  the  diazo-  and  azo-compounds  are  to  be  regarded  as 
products  of  the  incomplete  reduction  of  nitro-compounds. 
When  nitrobenzene  is  treated  with  nascent  hydrogen  the 
final  product  of  the  action  is  aniline,  the  process  being 
similar  to  the  reduction  of  nitric  acid  to  hydroxylamine : — 

C6H5 

6H    =    £NH        +     2H20; 
O 


(OH 

+     6H    =    N-JH         +     2H2O 


NO 

(.0 


BENZENE  DERIVATIVES.  259 

But  just  as  intermediate  products  are  obtained  in  the 
reduction  of  nitric  acid,  viz.,  N2O3,  NO,  N2O,  and  N2,  so 
intermediate  products  are  obtained  in  the  reduction  of  the 
nitro-compounds.  The  relations  between  the  end-products 
and  the  intermediate  products  are  shown  in  the  following 
table:— 

Nitro-compound,  R.NO2. 

.    ^~^\r 
Azoxy-compound,  /O. 


Azo-compound,  R  —  N—  N  —  R. 
Hydrazo-compound,  R  —  NH  —  NH  —  R. 
Amido-compound,  R  —  NH2. 

PHENYLMETHANES. 

The  homologues  of  benzene  are  of  two  kinds,  as  has 
been  pointed  out.  They  are  obtained  either  by  introducing 
one,  two,  or  more  methyl  groups  into  benzene,  or  by 
introducing  homologous  residues  of  marsh-gas  hydrocar- 
bons into  benzene.  Just  as  methyl  groups  can  be  intro- 
duced into  benzene,  so  also  the  group  phenyl,  C6H5,  can 
be  introduced  into  methane.  Thus,  the  hydrocarbons  might 
be  formed  :  — 

Phenyl-methane,  CH3.C6H5  (identical  with  toluene)  ; 

Diphenyl-methane,         CH2(C6H5)2  ; 
Triphenyl-methane,        CH(C6H5)3;  and 
Tetraphenyl-methane,    C(C6H5)4. 

Of  the  last  three  members,  tetraphenyl  methane  is  not 
known;  and  triphenyl-methane  has  been  most  carefully 
studied.  It  is  the  mother-substance  of  two  important  groups 
of  compounds,  the  aniline  dyes  and  the  phthaleins. 

The  hydrocarbon  is  easily  obtained  by.  bringing  chloro- 
form, CHC13,  and  benzene,  C6H6,  together  in  the  presence 
of  aluminium  chloride  (reaction  of  Fried  el  and  Crafts). 
In  some  way  not  understood,  the  chloride  causes  the  two 
to  act  upon  each  other  as  represented  in  the  equation 

CHC13    +     3C6H6    =    CH(C6H5)3    +     3HC1. 

Rosaniline  and  Pararosaniline  —  The  aniline  dyes  are 
for  the  most  part  comparatively  simple  derivatives  of 
rosaniline  and  pararosaniline.  Pararosaniline  is  formed  by 


260    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 
oxidizing  a  mixture  of  aniline,  C6H5.NH2,  and  paratoluidine, 


\ 


NH2 


,  as  represented,  thus  :  — 


2C6H7N  +    C7H9N   +  3O  =  C19H19N3O  +  2H2O. 


67  79  19193  2 

Aniline.  Toluidine.  Pararosaniline. 

Rosaniline  is  formed  in  a  similar  way  :  — 
C6H7N  +  2C7H9N  +  3O  =  C20H17N3O  +  2H2O, 

Aniline.  Toluidine.  Rosaniline. 

Pararosaniline  has  been  made  from  triphenyl-methane 
by  the  following  reactions  which  show  the  relations  be- 
tween the  two  compounds  :  — 

By  treatment  with  nitric  acid  the  hydrocarbon  yields  a 
trinitro-derivative.  The  same  product  is  formed  by  bring- 
ing chloroform  and  nitro-benzene  together  in  the  presence 
of  aluminium  chloride,  thus  :  — 

CHC13  +  3C6H/NO2)  =  CH(C6H4.NO2)3  +  3HC1. 

Trinitro-triphenyl-methane. 

By  reduction  the  trinitro-product  is  converted  into  the 

C6H4.NH, 
corresponding   triamido-  derivative,  CH  -j  C6H4.NH2      = 


C19H19N3,  By  gentle  oxidation  this  compound,  known  as 
pamleucaniline,  is  converted  into  pararosaniline.  Para- 
rosaniline has  the  constitution  represented  by  the  formula 

fC6H,NH2 
C(OH)  <  C6H4.NH2  ;  or  it  is  triamido-triphenyl  carbinol. 

(.C6H4.NH2 

The  salts  of  this  compound  are  formed  by  addition  of  one 
molecule  of  the  acid  and  elimination  of  one  molecule  of 
water.  Thus  the  hydrochloric  acid  salt  is  formed  accord- 
ing to  this  equation  :  — 


(QeH^NHr  fC6H4.NH2  fC6H4.NH2 

C(OH)  4  C,H4.NH2  +  HC1  ==  C     C6H4  NH2        or  CC1  4  C6H4.NH2 
|.C6H4.NH2  I    (C6H4.NH.HC1  (C6H4.NH2 


Similarly,  rosaniline  is  represented  by  the  formula 


C6H4.NH2 

Its  salts  are  analogous  to  those  of  pararosaniline, 


BENZENE  DERIVATIVES.  261 

Phthaleins.  —  The  phthaleins  are  formed  by  treating  a 
mixture  of  phthalic  anhydride,  C8H4O3,  and  a  phenol 
with  a  dehydrating  agent.  The  simplest  representative 
of  the  class  is  phenol-phthalein,  which*  is  made  by  heating 
a  mixture  of  ordinary  phenol  and  phthalic  anhydride  with 
sulphuric  acid  :  — 

2C6H60     +     C8H403    =    C20HU04     +     H20. 

Phenol.  Phthalic  Phenol-phthalein. 

anhydride. 

The  phthaleins  are  derivatives  of  triphenyl-methane,  as 
has  been  shown  by  the  following  transformations  :— 

The  chloride  of  phthalic  acid,  C8H4O2C12,  when  treated 
with  benzene  in  the  presence  of  aluminium  chloride,  yields 
a  product  known  as  diphenyl-phthalide  :  — 

C8H402C]2  +  2C6H6  =  C8HA(C6H5)2  +  2HC1. 

Phthalyl  chloride.  Diphenyl-phthalide. 

When  boiled  with  sodium  hydroxide,  diphenyl-phthalide 
is  transformed  into  an  acid,  triphenylcarbinol-carbonic 
acid  :  — 


When  treated  with  zinc  dust  this  acid  loses  oxygen  :  — 

COOH         TT  COOH    ,    TT  ^ 

" 


The  product,  triphenylmethane-carbonic  acid,  when  dis- 
tilled with  baryta,  loses  carbon  dioxide  and  yields  triphenyl- 
methane.  It  is  hence  to  be  regarded  as  a  simple  carboxyl 

("  QHs 
derivative  of  triphenylmethane,  CH  4  C6H5 

(  C6H4.COOH 

In  accordance  with  this  conclusion,  the  other  substances 
in  the  series  must  be  represented  by  the  following  for- 
mulas :  Triphenylcarbinol-carbonic  acid, 

f  C1  TT 

C(OH)  )  C66H55  ;  diphenyl-phthalide,  C 

(C6H4.COOH 


12* 


262    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

Phenol-phthalein  is  derived  from  the  last  compound  by 
the  introduction  of  two  hydroxyls,  and  is  represented  by 

C6H4.OH 
the  formula  C  -j  C6H4.OH  =  C20H14O4. 

C6H4.CO 


Phenylethylene.  —  Ethylbenzene  or  phenylethane,  C2H5. 
C6H5,  can  be  converted  into  hydrocarbons  containing  two 
or  four  hydrogen  atoms  less,  by  means  of  the  same  reac- 
tions as  those  made  use  of  for  converting  ethane  into 
ethylene  and  acetylene.  Thus,  from  ethane,  bromethane, 
C2H5Br,  is  made,  and  this,  when  treated  with  an  alcoholic 
solution  of  potassium  hydroxide,  loses  hydrobromic  acid, 
yielding  ethylene,  C2H5Br  —  HBr=C2H4;  and  when  di- 
bromethane,  C2H4Br2,  is  treated  in  a  similar  way,  it  yields 
acetylene,  C2H4Br2  —  2HBr=C2H2.  In  like  manner 
bromphenylethane,  C6H5.C2H4Br,  yields  phenylethylene, 


.Br—  HBr  =  C6H5.C2H3. 

Phenylethylene  is  commonly  called  styrene.  It  is  the 
mother-substance  of  the  compounds  of  the  indigo  group. 
The  relation  of  indigo  blue  to  this  hydrocarbon  has  been 
established  through  the  labors  of  Von  Baeyer.  Some  of  the 
derivatives  are  simpler  and  can  be  explained  more  easily. 

Cinnamic  acid  has  been  shown  to  be  phenylacrylic  acid, 
C6H—  CH—  CH.COOH,  that  is,  it  is  the  simple  carboxyl 
derivative  of  phenylethylene,  C6H5—  CH=rCH2.  When 
treated  with  nitric  acid  cinnamic  acid  yields  two  mono- 
nitroderivatives,  one  of  which  belongs  to  the  ortho-series, 

/C2H2COOH 
C6H5( 

XN02 

This  compound  can  easily  be  changed  to  indigo-blue,  but 
the  intermediate  reactions  are  complicated. 

When  indigo-blue  is  oxidized  it  is  converted  into  isatine, 
C8H6NO2.  A  simple  synthesis  of  isatine  has  been  effected, 
which  shows  clearly  what  its  constitution  is. 

(  (^OOTT 

Orthoamidobenzoic  acid,  C6H4  -j  -^  jj         ,  yields  a  chlo- 


i 

ride,  C6H4  •<  -V  .  The  cyanogen  group  can  be  substituted 


BENZENE  DERIVATIVES.  263 

for  the  chlorine  in  this  compound,  and  then  converted  into 

( r^o  POOTT 
carboxyl.     Thus,  an  acid,  C6H4  j  J£gV         ,   is   obtained. 

This  loses  water  very  readily,  and  the  anhydride  thus 
formed,  which  is  represented  by  the  formula 

(CO.  CO 
CJEL  4  I     ,  is  isatine. 

(NH- 

Furfuran. — Furfuran  is  formed  from  brompyromucic 
acid.  It  has  the  composition  expressed  by  the  formula 
C4H4O.  Like  benzene,  it  forms  addition-products,  and  is 
therefore  probably  unsaturated,  just  as  ethylene  and  acety- 
lene and  their  derivatives  are  unsaturated.  It  is  not  an 
alcohol  nor  an  aldehyde  nor  a  ketone.  Taking  all  the 
facts  known  in  regard  to  it  into  consideration,  the  most 
probable  hypothesis  suggested  concerning  its  constitution 

CH— CH 

II         II 
is  that  expressed  by  the  formula  CH    CH  .     This  repre- 

\/ 
0 

sents  a  compound  similar  to  benzene  in  respect  to  the  ring 
structure.  It  is  benzene  in  which  an  oxygen  atom  has 
taken  the  place  of  two  of  the  CH  groups.  Perfectly  satis- 
factory evidence  of  the  correctness  of  this  view  has  not  been 
furnished,  though  the  analogy  between  furfuran,  pyrrol,  and 
thiophene,  the  structure  of  which  has  been  more  thoroughly 
investigated,  renders  the  view  highly  probable. 

There  are  a  number  of  derivatives  of  furfuran  known, 
among  which  are  furfural,  C4H3O.COH,  which  is  formed 
by  the  dry  distillation  of  several  of  the  carbohydrates,  and 
pyromucic  acid,  C4H3O.COOH,  which  is  formed  by  the  dry 
distillation  of  mucic  acid. 

Pyrrol. — This  compound  with  some  of  its  homologues  is 
found  in  bone  oil  and  coal-tar  oil.  It  is  formed  by  the 
distillation  of  the  ammonium  salts  of  mucic  and  saccharic 
acids  and  by  reduction  of  succinimide  with  zinc  dust.  One 
hydrogen  atom  in  the  compound  evidently  differs  from  the 
others,  as  potassium  can  be  introduced  in  place  of  only  one 
of  them  when  pyrrol  is  treated  with  this  metal.  Further, 
acetyl,  C2H3O,  can  be  introduced  in  place  of  only  one  hy- 


succinimide,  |  /NH,  suggests  the  formula  CH     CH, 

CH2.< 


264     PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

drogen  atom.  These  facts  make  it  probable  that  pyrrol 
contains  the  iinide  group,  and  its  formation  by  reduction  of 

CH— CH 
CH,.COV  ii         » 

XXX 

NH 

according  to  which  furfuran  and  pyrrol  are  analogous 
compounds,  bearing  to  each  other  the  same  relation  that 
succinic  anhydride  bears  to  succinimide  : — 

CH— CH2  CH2— CH2 

CO      CO  00      CO 

\/  \/ 

O  NH 

Succinic  anhydride.  Succinimide. 

CH— CH  CH— CH 

II        II  II        II 

CH    CH  CH    CH 

\/  \/ 

O  NH 

Furfuran.  Pyrrol. 

Thiophene. — In  the  benzene  obtained  from  coal-tar  oil 
there  is  contained  a  substance  of  the  composition  C4H4S, 
which  is  remarkably  like  benzene  in  all  its  properties  ;  and 
its  derivatives  also  exhibit  the  same  resemblance  to  those  of 
benzene.  This  is  thiophene.  The  resemblance  to  benzene 
suggests  at  once  that  it  probably  has  a  similar  constitution, 

CH— CH 

II         II 
as  represented  in  this  formula,  CH     CH.     According  to 

S 

this,  thiophene  is  not  only  related  to  benzene,  but  to  fur- 
furan and  pyrrol.  The  relation  to  furfuran  and  pyrrol  is 
clearly  shown  by  a  number  of  reactions.  There  is  a  sub- 
stance which,  when  treated  with  dehydrating  agents,  yields 
a  furfuran  derivative;  when  this  same  substance  is  treated 
with  an  alcoholic  solution  of  ammonia  it  yields  a  pyrrol 
derivative;  and  when  treated  with  phosphorus  pentasul- 
phide  it  yields  a  thiophene  derivative.  Further,  thiophene 
is  formed  when  succinic  anhydride  is  treated  with  phos- 


BENZENE  DERIVATIVES.  265 

phorus  pentasulphide ;  while,  as  was  above  stated,  pyrrol 
is  formed  from  succinimide  by  reduction.  The  study  of 
the  isomeric  substitution-products  obtained  from  thiophene 
has  only  tended  to  confirm  the  formula  above  given.  The 
ex  istence  of  two  isomeric  mono-substitution-products  is  in- 
dicated, and  only  two  have  been  obtained,  although  many 
efforts  have  been  made  to  make  a  third. 

NAPHTHALENE. 

The  hydrocarbon  naphthalene  has  the  formula  C10H8. 
It  is  believed  to  be  formed  by  the  union  of  two  benzene 
residues  in  such  a  way  that,  while  the  compound  contains 
the  two  residues,  two  of  the  carbon  atoms  are  common  to 
both  residues.  This  is  the  fundamental  idea  in  the  pre- 
vailing view  regarding  the  constitution  of  naphthalene.  It 
is  expressed  thus  : — 

H  H 

C  C 


Assuming  the  formula  for  benzene  to  be  correct,  this  is 
the  only  possible  formula  for  naphthalene.  It  is  based  on 
the  following  facts  :  There  is  a  derivative  of  naphthalene 
known  as  dichlornaphthoquinone,  which  has  the  formula 
C10H4C12O2.  When  this  substance  is  oxidized  it  yields 
phthalic  acid,  which  is  a  di-substitution-product  of  ben- 
zene. We  see  thus  that  those  carbon  atoms  in  dichlor- 
naphthoquinone which  are  not  in  combination  with  chlo- 
rine form  a  benzene  nucleus,  so  that  we  might  write  the 
formula  of  the  compound  C6H4.C4C12O2.  This  formula 
does  not  tell  us  in  what  manner  the  atoms  C4C12O2  are 
united  ;  but  by  the  aid  of  another  experiment  this  can  be 
determined. 

When  dichlornaphthoquinone  (the  substance  used  in  the 
preceding  experiment)  is  treated  with  phosphorus  penta- 
chloride,  it  is  converted  into  pentachlornaphthalene,  the 


266    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

formula  of  which,  according  to  what  was  said  above,  is 
C6H3C1.C4C14.  By  analogy  we  should  expect  this  com- 
pound by  oxidation  to  yield  monochlorphthalic  acid ;  it, 
however,  yields  tetrachlorphthalic  acid.  This  shows  that 
the  four  carbon  atoms  which  are  in  combination  with  the 
four  chlorine  atoms  form  part  of  a  benzene  ring,  as  well 
as  the  other  carbon  atoms  of  naphthalene.  It  is  thus 
proved  that  in  naphthalene  there  are  two  benzene  rings. 
The  only  formula  that  agrees  with  this  fact  is  the  one 
above  given. 

The  following  facts  lead  to  the  same  conclusion :  Nitro- 
naphthalene  when  oxidized  yields  nitro  phthalic  acid. 
Therefore  the  part  of  the  compound  in  which  the  nitro- 
group  is  contained  is  a  benzene  ring.  When  the  nitro  - 
compound  is  reduced  amido-naphthalene  is  formed,  and 
this  by  oxidation  is  converted  into  phthalic  acid.  It 
appears  from  this  that  there  must  be  another  benzene  ring 
in  naphthalene  besides  that  in  which  the  nitro-  or  amido- 
group  is  contained. 

The  derivatives  of  naphthalene  resemble  those  of  ben- 
zene, and  much  that  has  been  said  concerning  the  latter 
holds  good  in  regard  to  the  former.  Substituting  groups 
or  elements  can  take  the  place  of  all  the  hydrogen  atoms 
of  naphthalene,  and  thus,  as  will  readily  be  seen,  a  large 
number  of  substitution-products  can  be  obtained.  The 
possibilities  of  isomerism  are  greater  in  the  case  of  naph- 
thalene than  in  the  case  of  benzene,  but  the  principles 
underlying  the  isomerism  are  essentially  the  same  as  those 
which  have  already  been  considered  in  connection  with  the 
isomeric  substitution-products  of  benzene. 

According  to  the  researches  of  Bamberger,  naphthalene 
does  not  consist  of  two  benzene  rings,  but  it  is  so  con- 
stituted that  when  addition  to  either  of  the  rings  takes 
place  the  other  ring  becomes  a  benzene  ring.  This  con- 
clusion is  based  upon  elaborate  investigations  of  the  hydro- 
gen addition-products  of  naphthalene  and  its  derivatives. 
Bamberger  suggests  that  the  constitution  may  be  expressed 
by  this  formula; — 


BENZENE  DERIVATIVES. 
CH          CH 


267 


CH 


CH 


CH 


It  will  be  seen  that  some  of  the  bonds  of  the  carbon  atoms 
are  directed  toward  the  centre  of  the  ring,  but  are  not 
connected  with  the  others  which  act  in  the  same  direction. 
It  may  fairly  be  asked  whether  the  facts  known  warrant  a 
distinction  between  the  above  formula  and  this : — 


CH 


CH 


CH 


And  to  this  question  only  a  negative  answer  can  be  given. 
Taking  the  second  form  then,  how  does  it  explain  the  fact 
brought  out  by  a  study  of  the  addition-products  of  naph- 
thalene? In  the  first  place,  it  represents  a  compound  con- 
sisting of  two  rings  of  the  same  constitution  ;  second,  these 
rings  are  not  benzene  rings;  third,  should  addition  take 
place  to  either  ring  the  other  ring  would  become  a  benzene 
ring,  as  shown  in  the  formula  below  : — 


CH 


CH 


CH 


CH 


CH2 


In  this,  the  ring  to  the  left  has  plainly  the  constitution  of 
a  benzene  ring,  as  represented  by  Claus,  and  held  by  Von 
Baeyer  to  be  the  most  probable. 


268    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 


Pyridine  and  Quinoline. — A  number  of  bases  of  the 
general  formula  CnH2n_5N  have  been  isolated  from  bone 
oil  and  made  in  other  ways.  The  simplest  one  is  pyridine. 
C6H5N.  The  others  are  homologous  with  it.  Pyridine  is 
a  very  stable  compound,  differing  markedly  in  its  chemical 
conduct  from  the  common  ammonium  bases.  By  oxidation 
its  methyl  derivatives  yield  corresponding  carbonic  acids, 
just  as  the  analogous  derivatives  of  benzene  yield  carbonic 
acids.  For  example:  toluene  or  methyl  benzene,  C6H5.CH3, 
yields  ben  zoic  acid,  C6H5  COOH.  So,  also,  picoline  or 
methyl-pyridine,  C5H4(CH3)N,  yields  pyridine-carbonic 
acid,  C5H4N.COOH,  etc.  While  the  facts  known  concern- 
ing pyridine  are  not  sufficient  to  justify  the  expression  of  a 
very  decided  opinion  regarding  its  structure,  they  neverthe- 
less speak  in  favor  of  a  structure  similar  to  that  of  benzene. 
If  one  of  the  CH  groups  of  benzene  is  replaced  by  a  nitro- 
gen atom,  the  resulting  compound  would  have  the  formula 


=  C5H5N. 


CH 


This  formula  represents  the  working  hypothesis  regard- 
ing the  structure  of  pyridine.  Thus  far,  the  cases  of 
isomerism  studied  are  in  accordance  with  this  formula. 
There  are,  for  example,  three  mono-methyl  derivatives  of 
pyridine,  and  three  corresponding  mono-carbonic  acids. 
The  formula  suggests  the  existence  of  these  three  varieties. 
The  three  methyl  derivatives  are  represented  thus : — 


H 


C— CH3  HC 


C— CH,  HC, 


CH 


N 
Ortho-picoline. 


N 
Meta-picoline. 


N 
Para-picoline. 


BENZENE  DERIVATIVES. 


269 


Quinoline,  C9H7N,  is  the  first  member  of  a  series  of  bases 
of  the  general  formula  CnH2n_nN,  which  somewhat  re- 
semble the  pyridine  bases.  Quinoline  is  believed  to  bear 
a  relation  to  naphthalene  similar  to  that  which  pyridine 
bears  to  benzene — a  conception  represented  by  the  formula 


=  C9H7N 


The  method  of  preparation  which  perhaps  most  easily 
admits  of  interpretation  is  the  following : — 

Cinnamic  aldehyde  has  the  constitution  represented  by 
the  formula  C6H5— CH= CH—  COH.  The  ortho-amido 
derivative  of  this  aldehyde  has  the  formula 


HC 


HC 


This  loses  water  easily  and  yields  quinoline.  As  will 
easily  be  seen,  the  loss  of  water  in  the  simplest  way  would 
lead  to  a  substance  of  the  formula 


CH 


CH 


HC 


HC 


e 


c 


CH 


CH 


C 
H 


N 


270    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

The  question  as  to  the  structure  of  pyridine  and  of 
quinoline  is  intimately  connected  with  the  question  as  to 
the  structure  of  benzene  and  of  naphthalene.  It  is  highly 
probable  that,  if  the  Glaus  formula  for  benzene  is  correct, 
all  the  other  substances  mentioned  have  a  similar  structure, 
though  this  does  not  necessarily  follow. 


ANTHKACENE. 

Anthracene,  like  naphthalene  and  benzene,  is  the  mother- 
substance  of  a  large  group  of  compounds.  Its  formula  is 
CUH10.  In  regard  to  its  constitution,  the  view  is  held  that 
it  consists  of  two  benzene  residues,  C6H4,  held  together 

by  means  of  the  group   HC — CH,  each  carbon  atom  of 
which  is  united  to  both  benzene  residues,  thus : — 


This  formula  is  based,  in  the  first  place,  on  the  similar- 
ity in  the  chemical  conduct  of  benzene,  naphthalene,  and 
anthracene.  Further,  a  synthesis  of  anthracene  has  been 

effected  from  orthobrombenzyl  bromide,  C6H4  j  ^^   . 

When  this  compound  is  treated  with  sodium,  one  of  the 
products  is  anthracene.  The  reaction  may  be  represented 
thus :— 


BENZENE  DERIVATIVES. 


271 


+  4NaBr  -f  2H. 


CH 


This  reaction  not  only  throws  light  upon  the  general 
character  of  anthracene,  but  it  shows  further  that  the  link- 
ing groups  CH  are  connected  with  the  benzene  residues  in 
the  ortho-position. 

The  formula  given  shows  the  relation  between  an- 
thracene and  anthraquinone,  which  appears  to  be 


C6H4 


C6H4.     The  latter  formula  best  explains  the 


formation  of  anthraquinone  from  benzoic  acid,  and  the 
formation  of  benzoic  acid  from  anthraquinone.  The  former 
transformation  is  represented  thus  :  — 


C6H4  H    CO    OH 


/\ 


=  C6H4 


CO 


2H20. 


C6H4  H]  CO  |  OH  | 

2  molecules  benzoic  acid. 


Anthraquinone. 


The  formation  of  anthraquinone  from  anthracene  should 
then  be  represented  thus  : — 

,CH,  ,CO, 


\ 


CO 


272    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

According  to  these  interpretations,  anthraquinone  is  a 
double  ketone.  It  has  been  suggested,  further,  that  all 
quinones  are  double  ketones.  (See  Quinones.) 


Other  hydrocarbons  allied  to  naphthalene  are  pyrene, 
chrysene,  and  phenanthrene.  These  have  not  been  very 
fully  investigated  as  compared  with  naphthalene  and  an- 
thracene themselves.  All  of  these  three  undoubtedly  con- 
tain benzene  residues  as  essential  parts  of  their  molecules, 
but  there  is  still  some  doubt  in  regard  to  the  manner  in 
which  these  residues  are  united. 


RETROSPECT. 

A  study  of  the  preceding  chapters  on  constitution  will 
show  that  no  absolute  meaning  is  to  be  attached  to  the 
word.  Constitutional  formulas  are  those  which  suggest 
certain  reactions  and  recall  analogies.  The  formula 
CH3 — OH  does  not  mean  that  hydroxyl  (OH)  is  neces- 
sarily present  in  the  compound,  or  that  CH3  is  present,  but 
that  the  different  parts  of  the  compound  bear  such  rela- 
tions to  each  other  that  when  the  compound  is  decomposed 
it  acts  as  if  the  parts  were  united  as  the  formula  indicates. 
The  formula  suggests  possibilities;  it  may  not  represent 
realities. 

The  methods  thus  far  considered  for  determining  the 
constitution  of  compounds  are  chemical  methods.  They 
depend  upon  the  study  of  chemical  conduct.  If  we  find 
that  two  compounds  conduct  themselves  in  the  same  gen- 
eral way,  the  formulas  call  this  analogy  to  mind.  If  we 
assume  a  certain  grouping  in  the  one,  we  must  assume  a 
similar  grouping  in  the  other. 

The  question  whether  a  study  of  the  physical  proper- 
ties of  compounds  can  throw  any  light  upon  the  constitu- 
tion of  the  compounds  now  naturally  suggests  itself.  A 
great  deal  of  attention  has  been  paid  to  the  subject  of  late, 
and  a  brief  account  of  some  of  the  methods  employed  and 
the  results  reached  will  now  be  .given. 


PHYSICAL  METHODS.  273 


CHAPTER   XVII. 

PHYSICAL   METHODS    FOR    THE    DETERMINATION    OF   THE 
CONSTITUTION   OF   CHEMICAL   COMPOUNDS. 

General. — The  study  of  chemical  reactions  makes  it 
possible  to  determine  the  constitution  of  compounds  in  the 
sense  in  which  the  word  constitution  has  been  explained ; 
that  is,  it  makes  it  possible  to  draw  conclusions  in  regard 
to  the  connections  existing  between  the  different  atoms  in 
molecules.  The  formulas  expressing  the  results  of  the 
study  are  based  upon  a  few  fundamental  conceptions  in 
regard  to  the  way  in  which  molecules  are  made  up.  For 
example,  the  formulas  of  all  the  hydrocarbons  of  the  me- 
thane series  are  based  upon  the  conception  that  in  the  mole- 
cule of  marsh  gas  four  atoms  of  hydrogen  are  combined 
in  the  same  way  with  an  atom  of  carbon,  and  the  further 
conception  that  in  ethane  the  two  carbon  atoms  are  in  com- 
bination with  each  other  and  also  with  three  hydrogen 
atoms  each.  Given  these  conceptions,  and  the  theory  of 
the  whole  series  follows.  The  formulas  of  the  members 
of  the  series  are  then  the  expressions,  in  terms  of  the 
theory,  of  the  facts  learned.  Further,  we  find  the  same 
ideas  underlying  the  prevailing  views  regarding  the  struc- 
ture of  all  carbon  compounds.  So,  too,  when  a  number 
of  compounds  are  represented  by  formulas  in  which  double 
and  triple  lines  occur,  all  that  is  meant  by  them  is  that 
those  in  which  the  double  lines  occur  are  like  ethylene,  and 
those  in  which  the  triple  lines  occur  are  like  acetylene ;  and 
the  question  as  to  what  is  the  actual  condition  between  the 
atoms  thus  represented  remains  an  open  one.  The  same 
thing  is  true  in  regard  to  the  benzene  derivatives.  By  a 
thorough  study  of  benzene  a  conception  is  formed  in  regard 
to  the  constitution  of  this  compound.  This  conception  is  a 
result  of  an  extension  of  the  ideas  already  formed  in  study- 
ing the  methane  derivatives.  Whenever  a  formula  like 
that  of  benzene  is  seen  it  is  recognized  at  once  as  the  sign 


274    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

of  the  benzene  condition  and  the  record  of  certain  reactions. 
With  the  question  of  the  physical  structure  of  the  molecules 
the  views  in  regard  to  chemical  constitution  have  nothing 
directly  to  do.  Indeed,  it  does  not  appear  probable  that 
the  study  of  chemical  reactions  alone,  that  is,  of  the  decom- 
positions and  methods  of  formation  of  compounds,  can  ever 
lead  to  definite  views  in  regard  to  the  physical  structure, 
if  by  physical  structure  is  meant  the  arrangement  of  the 
atoms  in  space  and  the  relations  which  they  bear  to  one 
another.  The  only  hope  of  gaining  views  upon  this  subject 
lies  in  a  study  of  the  physical  phenomena  attending  chemi- 
cal reactions ;  in  an  application  of  physical  methods  to  the 
study  of  chemical  problems.  But  the  application  of  phy- 
sical methods  to  the  study  of  chemical  problems  will  not 
necessarily  touch  the  question  of  physical  structure;  and, 
indeed,  the  chief  results  thus  far  obtained  in  this  field  have 
simply  had  reference  to  the  determination  of  a  few  points 
in  connection  with  the  chemical  constitution.  It  has  been 
found  that  in  certain  physical  properties  saturated  com- 
pounds differ  from  unsaturated,  and  that,  of  the  unsaturated 
compounds,  those  with  double  linkage  differ  from  those  with 
triple  linkage.  Therefore,  by  taking  these  differences  into 
account,  it  is  possible  in  some  cases  to  determine  whether  a 
substance  is  saturated  or  unsaturated.  Beyond  this  the 
application  of  physical  methods  has  not  advanced  so  far  as 
the  determination  of  constitution  is  concerned,  except  in  a 
very  few  cases  which  will  be  referred  to  under  thermal 
methods. 

Specific  Volume. — By  the  specific  volume  or  molecular  vol- 
ume of  a  substance  is  meant  a  value  expressed  by  a  figure 
obtained  by  dividing  the  specific  gravity  of  the  substance 
in  the  form  of  liquid  into  the  molecular  weight.  Thus,  the 
specific  gravity  of  ordinary  alcohol  at  the  boiling  tempera- 
ture is  0.736 ;  the  molecular  weight  is  24  -f-  6  -f-  16  =  46  ; 

46 

the  specific  volume  is  TTW^  =  62.5. 
U.7oo 

A  study  of  the  specific  volumes  of  a  large  number  of 
substances  has  shown  that  a  close  connection  exists  between 
the  figures  and  the  constitution  of  the  substances.  The 
specific  gravities  must  be  determined  under  strictly  analo- 
gous conditions,  in  order  that  they  may  be  comparable. 
The  figures  used  are  those  found  at  the  boiling-points  of  the 


UNIVERSITY 
PHYSICAL  MTgwrftnS^ALIFOaa&ffo 

liquids.     Kopp  first  pointed  out  that  the  following  definite 
relations  exist : — 

1.  In  many  instances  differences  in  specific  volume  are 
proportional  to   differences  in  corresponding  chemical  for- 
mulas.    Thus  a  difference  of  CH2  in  an  homologous  series 
corresponds  to  a  difference  of  about  22  in  the   specific 
volume,  or  (CH?)  x  =  22  x.     On  comparing  the  specific 
volumes  of  haloid  compounds  of  similar  constitution  it  is 
seen  that  the  substitution  of  n  atoms  of  bromine  for  an 
equal  number  of  chlorine  atoms  increases  the  specific  volume 
by  5n. 

2.  Metameric  liquids  have,  as  a  rule,  the  same  specific 
volume.     Exceptions  are  exhibited  by  certain  oxygen  and 
sulphur  compounds. 

3.  The  substitution  of  an  atom  of  carbon  for  two  of  hydro- 
gen makes  no  alteration  in  the  specific  volume  of  members  of 
certain  groups  of  organic  liquids. 

On  the  basis  of  his  observations  Kopp  was  able  to  cal- 
culate certain  fundamental  values  for  the  specific  volumes 
of  the  elements  in  combination.  These  values  are,  as  a 
rule,  constant  for  any  particular  element :  thus,  carbon  has 
invariably  the  value  11 ;  hydrogen  that  of  5.5.  Exceptions 
are  observed  in  the  case  of  the  chemical  analogues  oxygen 
and  sulphur.  Each  of  these  elements  has  two  values,  de- 
pending, it  would  seem,  on  its  mode  of  combination,  or  on 
its  relation  to  the  remaining  atoms  in  the  molecule.  For 
example:  acetone  and  allyl  alcohol  have  the  empirical 
formula  C3H4O,  but  the  specific  volume  of  acetone  is  78.2, 
whilst  that  of  allyl  alcohol  is  73.8.  As  expressed  in  the 
ordinary  formulas  the  affinities  of  the  oxygen  in  acetone 
are  wholly  satisfied  by  the  carbon,  whereas  in  allyl  alcohol 
a  moiety  of  the  combining  value  would  seem  to  be  satisfied 
by  carbon,  and  the  remainder  by  hydrogen.  It  appears, 
then,  that  when  oxygen  is  united  to  an  element  by  both  its 
affinities  its  specific  volume  is  12.2;  when  it  is  attached  by 
only  one  combining  unit  its  specific  volume  is  7.8.  The 
corresponding  values  for  sulphur  are  28.6  and  22.6.* 

Assuming  the  conclusions  of  Kopp  to  be  correct,  a 
method  is  given  for  determining  in  what  condition  oxygen 
is  present  in  a  compound.  An  illustration  will  make  the 
application  of  the  method  clear.  Suppose  a  compound  of 

*  See  Thorpe,  Journal  of  the  Chemical  Society,  xxxvii.  141. 


276    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

the  formula  C2H4O  has  the  specific  gravity  0.773.  The 
specific  volume  is  -g- -f-f^  =  56.9.  Calculating  the  specific 
volume  according  to  Kopp's  rule,  and  accepting  his  values 
for  carbon  and  hydrogen,  viz.,  C  — 11;  H  =  5.5,  we  have 
V(C2H4O)  =  2  x  11  +  4  x  5.5  -f-  x,  in  which  x  represents 
the  specific  volume  of  the  oxygen.  But  the  specific  volume 
of  the  compound  found  by  experiment  is  56.9  ;  hence  we 
have  2  x  11  +  4  x  5.5+ a;  =  66.9;  a  =  12.9. 

The  specific  volume  of  oxygen  in  the  compound  under 
consideration  is  thus  shown  to  be  12.9,  or  approximately 
the  value  assigned  to  oxygen  combined  with  carbon  by  two 
bonds.  Thus  the  presence  of  the  group  — CO —  is  shown 
by  this  method. 

A  thorough  examination  of  all  the  facts  on  record  bear- 
ing upon  the  subject  of  specific  volumes  has  led  Lossen  to 
conclude  that  the  rule  of  Kopp  is  not  strictly  true ;  that  the 
specific  volumes  of  the  elements  vary  somewhat  according 
to  the  class  to  which  the  compound  into  which  they  enter 
belongs.  He  considers  it  probable  that  certain  figures  will 
be  found  to  express  the  specific  volumes  of  the  elements  as 
they  occur  in  different  classes  of  compounds,  and  that  these 
figures  will  be  very  similar  to  those  given  by  Kopp. 

Molecular  Refraction. — The  study  of  the  refracting  power 
of  different  organic  liquids  has  led  to  results  of  interest, 
which  show  that  there  is  a  close  connection  between  this 
power  and  the  constitution  of  the  compounds.  In  order 
that  the  results  may  be  comparable  the  refraction-equiv- 
alent is  determined.  This  is  represented  by  the  expression 
P(^),  in  which  P  is  the  molecular  weight  of  the  compound, 
n  the  index  of  refraction,  and  d  the  density  of  the  com- 
pound. The  index  is  determined  for  four  different  lines  of 
the  spectrum  obtained  from  the  sodium  light,  and  from  the 
light  emitted  by  hydrogen  in  a  Geissler's  tube. 

Landolt  has  shown  that,  in  general,  substances  of  the 
same  composition  have  the  same  refraction-equivalent,  the 
value  of  this  equivalent  being  dependent  upon  the  number 
and  kinds  of  atoms  present  in  a  molecule,  rather  than  upon 
the  arrangement  of  these  atoms.  Each  atom  must  then 
have  its  own  refraction  equivalent,  and,  if  this  is  known, 
the  equivalent  for  a  substance  of  any  given  composition 
can  be  calculated.  In  order  to  determine  the  values  for 
carbon,  hydrogen,  and  oxygen,  Landolt  made  use  of  several 


PHYSICAL  METHODS.  277 

methods,  the  principle  of  which  will  appear  from  the  fol- 
lowing illustrations.  Two  compounds  were  compared  with 
each  other,  the  composition  of  which  differed  by  one  atom 
of  carbon,  two  atoms  of  hydrogen,  or  one  of  oxygen,  the 
difference  in  the  molecular  refraction  of  the  two  bodies 
giving  the  refraction-equivalent  of  the  element.  Thus,  the 
refraction-equivalent  (P^j1)  of  methyl  alcohol  is  13.17;  that 
of  aldehyde  is  18.58.  The  difference  in  composition  is  one 
carbon  atom.  It  is  hence  concluded  that  the  difference  in 
the  molecular  refraction  of  the  two  compounds  is  due  to  the 
one  carbon  atom,  or  that  the  atomic  refraction  of  carbon  is 
5.41.  In  the  same  way  aldehyde  and  ethyl  alcohol,  which 
differ  in  composition  by  two  hydrogen  atoms,  show  a  dif- 
ference in  molecular  refraction  of  2.12;  and  it  is  concluded 
that  the  atomic  refraction  of  hydrogen  is  half  of  2.12.  By 
comparing  aldehyde  and  acetic  acid,  which  differ  by  one 
atom  of  oxygen,  the  atomic  refraction  of  oxygen  is  found 
to  be  2.53. 

Difference  in  Composition  Q.  P(^jp)  Difference. 

Methyl  alcohol,  CH4O,  .             13.17  ) 

Aldehyde,          C2H4O,  .     18.58  }    5'41 

Difference  in  Composition  H2. 

Aldehyde,          C2H4O,  .         .     18.58  )     010 

Ethyl  alcohol,    C2H6O,  .         .     20.70  j     2'12 

Difference  in  Composition  Oj. 

Aldehyde,          C2H4O,  .        .     18.58  ) 

Acetic  acid,       C2H4O2,  .         .     21.11  j 

As  a  mean  of  a  large  number  of  observations,  the  fol- 
lowing values  were  obtained  for  the  atomic  refraction  of 
the  three  elements : — 

0  =  5;     H==1.3;     O  =  3. 

By  means  of  these  figures,  then,  the  refraction-equiva- 
lent of  any  given  compound  composed  of  these  three  ele- 
ments can  be  calculated.  On  comparing  the  calculated 
values  with  those  determined  by  experiment,  Briihl  found 
that  the  two  are  equal  in  a  large  number  of  cases,  but 
that,  in  a  number  of  other  cases,  the  values  found  by  ex- 
periment are  greater  than  those  calculated.  More  careful 
examination  of  the  subject  showed  that  all  the  compounds 

13 


278    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

that  form  exceptions  to  the  general  rule  belong  to  the  class 
of  unsatu rated  compounds,  and  that  all  saturated  com- 
pounds give  results  in  harmony  with  the  rule. 
It  was  shown  that : — 

1.  Univalent  elements  have  a  constant  atomic  refraction. 

2.  The  occurrence  in  a  compound  of  the  condition  known 
as  double  linkage  between  carbon  atoms  (see  pp.  102  and 
231)  causes  an  increase  of  2  in  the  value  of  the  molecular 
refraction  above  that  obtained  by  calculation. 

3.  The  presence  of  carbonyl,  CO,  causes  an  increase  in 
the  molecular   refraction  above  the  value  calculated  for 
oxygen  in  the  singly  linked  condition,  as  in  C — O — H. 

It  is  plain  that,  if  these  rules  are  well  founded,  a  method 
is  given  for  determining  whether  double  linkage  between 
carbon  atoms,  or  between  carbon  and  oxygen,  occurs  in 
compounds  under  examination. 

An  application  of  this  method  to  the  study  of  the  con- 
stitution of  benzene  led  Briihl  to  the  conclusion  that  there 
are  three  double  bonds  in  this  compound.  He  found  that 
the  molecular  refraction  is  greater  by  6  than  that  calcu- 
lated with  the  use  of  Landolt's  figures. 

Methods  dependent  upon  Determinations  of  the  Amount  of 
Heat  evolved  in  Chemical  Reactions  or  Thermal  Methods. — 
A  fact  which  constantly  impresses  the  observer  of  chemical 
phenomena  is  that  every  chemical  change  is  accompanied 
by  a  change  in  temperature.  In  general  the  direct  chem- 
ical combination  of  substances  causes  a  rise  in  temperature, 
while  decomposition  involves  an  absorption  of  heat.  For 
a  long  time  chemists  have  been  engaged  in  the  study  of 
the  heat-changes  caused  by  chemical  changes.  The  prin- 
cipal workers  in  this  field  have  been  Hess,  Favre  and 
Silbermann,  Berthelot,  and  J.  Thomsen.  The  method  of 
work  consists  in  allowing  known  weights  of  substances  to 
act  upon  each  other  in  carefully  constructed  vessels  called 
calorimeters,  so  that  all  the  heat  evolved  or  absorbed  can 
be  measured.  The  heat  unit  is  the  amount  of  heat  nec- 
essary to  raise  the  temperature  of  one  gram  of  water  one 
degree  centigrade.  This  is  called  a  calorie  and  is  repre- 
sented by  the  letter  c.  In  stating  the  results  of  the 
ther mo-chemical  study  of  a  reaction,  the  weights  of  sub- 
stances taken  into  consideration  are  the  number  of  grams 
of  each  that  correspond  to  their  atomic  or  molecular 


PHYSICAL  METHODS.  279 

weights.     Thus   the   formation  of  water  from    hydrogen 
and  oxygen  is  expressed  by  the  equation : — 

[H2,O]     =    68,360  c0 

This  means  that  when  2  grams  of  hydrogen  combine 
with  16  grams  of  oxygen  the  amount  of  heat  evolved  is 
68,360  calories.  It  is  obvious  that  observations  on  thermo- 
chemical  reactions  must  be  beset  with  difficulties.  Gener- 
ally other  changes  besides  the  chemical  changes  take  place, 
and  these  cause  thermal  changes,  and  it  is  by  no  means  a 
simple  matter  to  decide  what  part  of  the  total  change  is 
to  be  ascribed  to  the  chemical  changes.  When,  for  example, 
two  gaseous  substances  in  combining  give  a  liquid  or  a 
solid,  the  change  from  the  gaseous  to  the  liquid  or  solid 
state  causes  an  evolution  of  heat,  which  is  of  course  meas- 
ured with  that  caused  by  the  chemical  combination,  and, 
unless  by  special  measurements  the  amount  of  heat  thus 
evolved  is  determined,  tha  thermo-chemical  change  proper 
is  not  known.  Ingenious  methods  have  been  devised  for 
getting  over  the  difficulties,  and  for  a  large  number  of 
chemical  reactions  figures  have  been  obtained  of  which  it 
may  be  stated  that  they  express  analogous  facts,  While 
thermo-chemical  studies  have  unquestionably  been  of  bene- 
fit to  chemistry  and  promise  to  be  of  greater  benefit  in  the 
future,  the  generalizations  reached  by  following  this  line 
of  work  are  not  as  yet  of  such  character  as  to  have  a  direct 
bearing  upon  most  studies  of  chemical  phenomena.  This 
is  partly,  perhaps  mostly,  due  to  the  fact  that  the  prevail- 
ing hypotheses  in  regard  to  the  constitution  of  chemical 
compounds  and  the  nature  of  chemical  reactions,  although 
they  are  of  the  highest  value  and  have  been  incalculably 
helpful  to  the  science,  are  extremely  crude  when  regarded 
from  the  mechanical  point  of  view. 

Heat  of  Formation. — Among  the  thermo-chemical  meas- 
urements which  have  been  made  in  largest  number  is  that 
which  is  called  the  heat  of  formation.  By  this  is  meant 
the  heat  evolved  in  the  formation  of  a  compound.  This 
heat  of  formation  cannot,  however,  in  most  cases  be  meas- 
ured directly,  and  must  therefore  be  measured  indirectly. 
Thus,  for  example,  the  heat  of  formation  of  marsh-gas  can- 
not be  measured  directly.  In  such  a  case  the  measurement 
is  made  as  follows :  The  heat  generated  by  the  combustion 


280    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

of  carbon  is  measured ;  then  that  generated  by  the  com- 
bustion of  hydrogen,  and  the  two  are  added.  It  has  been 
found  that 


[C,O2]   =     96,960  c., 
[H2,0]  =--  1 


and2[H2,O]  =  136,720  c. 


Therefore  233,680  c.  is  the  heat  of  combustion  of 
the  constituents  of  a  molecule  of  marsh-gas.  Now  the  heat 
of  combustion  of  marsh-gas  itself  is  determined.  As,  in 
this  operation,  some  of  the  heat  evolved  is  used  in  sepa- 
rating the  carbon  and  hydrogen,  the  heat  of  combustion 
of  marsh-gas  is  less  than  that  of  its  constituents.  The 
following  equation  expresses  the  result  of  experiments : — 

[CH41OJ     =    211,930  c. 

The  difference  between  233,680  c.  and  211,930  c.,  which 
is  21,750  c.,  represents  the  amount  of  heat  absorbed  in 
separating  the  carbon  from  the  hydrogen.  But,  as  a  result 
of  a  great  many  measurements,  the  law  has  been  established 
that  the  heat  absorbed  in  the  decomposition  of  a  substance  is 
equal  to  the  heat  evolved  in  its  formation;  and  this  law  is 
also  included  in  the  general  laws  of  the  conservation  of 
energy.  Therefore,  the  heat  of  formation  of  marsh-gas  is 
21,750  c. 

Thomsen  has  determined  the  heat  of  formation  of  a  large 
number  of  hydrocarbons,  and  has  attempted  to  interpret 
the  results  in  terms  of  the  valency- atomic  hypothesis.  It 
would  lead  too  far  to  discuss  here  Thomson's  results  in 
detail.  Suffice  it  to  say,  he  believes  that  by  a  study  of  the 
heat  of  formation  of  various  hydrocarbons  such  as  methane, 
CH4,  ethane,  C2H4,  ethylene,  C2H4,  acetylene,  C2H2,  propane, 
C3H8,  and  propylene,  C3H6,  it  is  possible,  first,  to  draw  a 
conclusion  in  regard  to  the  heat  evolved  when  a  molecule 
of  gaseous  carbon  C^C  is  formed,  assuming  that  this  mole- 
cule contains  two  atoms.  This  is  reached  by  a  consideration 
of  the  heats  of  formation  of— 


2[C,HJ  = 


C2,H; 

c,,H6; 
:c2,H4; 


Between  the  thermal  values  of  these  reactions  there  is  a 
constant  difference,  and,  assuming  that  the  same  difference 


PHYSICAL  METHODS.  281 

holds  good  in  passing  to  the  next  member  of  the  series* 
viz.,  C2,  the  heat  of  formation  of  this  last  member  follows. 
Starting  with  these  data  and  with  the  known  heat  of  com- 
bustion of  carbon  and  of  carbon  monoxide,  values  are 
obtained  for  the  heat  evolved  when  two  carbon  atoms  unite 
by  single  linkage  (vx),  by  double  linkage  (v2),  and  by  triple 
linkage  (v3) ;  and,  further,  a  general  formula  is  deduced  for 
the  heat  of  formation  of  hydrocarbons.  In  this  formula 
the  term  2  v  enters.  This  represents  the  sum  of  the  values 
vi>  V2>  V3-  This  term  is  the  unknown  quantity.  Knowing 
then  the  number  of  single,  double,  and  triple  linkages  that 
occur  in  a  hydrocarbon,  it  is  possible  to  calculate  the  heat 
of  formation;  or,  knowing  the  heat  of  formation,  it  is  pos- 
sible to  determine  the  number  of  single,  double,  and  triple 
linkages  in  the  hydrocarbon.  It  is  by  an  application  of 
this  method  to  benzene  that  Thomsen  reached  the  conclu- 
sion that  in  this  hydrocarbon  there  is  no  double  linkage, 
but  only  single  linkage,  a  conclusion  which  is  in  harmony 


CH 

with  the  formula  |     ^|\     |          .      The  fact  that 

CH 

CH 

this  conclusion  is  directly  opposed  to  that  reached  by  a 
study  of  the  refracting  power  of  benzene  admonishes  us 
not  to  accept  either  without  further  investigation. 

It  will  be  observed  that,  even  though  this  method  were 
fully  established,  nothing  material  would  be  added  to  our 
knowledge  of  the  structure  of  chemical  compounds.  The 
method  would  merely  supplement  the  chemical  methods 
already  discussed,  and  enable  us  to  classify  hydrocarbons, 
and  perhaps  other  carbon  compounds,  according  to  the 
prevailing  chemical  hypotheses. 

Heat  of  Neutralization. — Studies  on  the  heat  of  neutral- 
ization of  acids  and  bases  have  led  to  some  conclusions 
which  may  eventually  prove  of  importance  in  connection 
with  questions  of  structure.  For  example,  Thomsen  found 


282    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

that  the  heat  of  neutralization  of  the  first  third  of  the 
hydrogen  in  phosphoric  acid  with  caustic  soda  and  that  of 
the  second  third  are  nearly  equal,  while  that  of  the  last 
third  is  much  less.  He,  therefore,  concluded  that  phos- 
phoric acid  is  not,  strictly  speaking,  a  tribasic  acid.  If 
this  conclusion  is  accepted,  it  ought  to  be  taken  into  con- 
sideration in  framing  our  views  regarding  the  constitution 
of  phosphoric  acid.  It  may  possibly  be  that  the  constitu- 

(  O— O— H 
tion  should  be  represented  by  the  formula  P  \  OH 

(OH 

Up  to  the  present,  however,  the  conclusions  regarding 
structure  reached  by  thermo-chernical  studies  are  not  gen- 
erally accepted  when  they  conflict  with  those  reached  by 
purely  chemical  studies. 

Other  Conclusions. — Among  other  conclusions  reached 
by  Thomsen  as  a  result  of  his  thermo-chemical  researches 
the  following  may  be  mentioned  : — 

1.  The  four  affinities  or  bonds  of  the  carbon  atom  are 
equivalent. 

2.  Pyridine  and  thiophene  have  no  double  linkages  be- 
tween the  carbon  atoms. 

3.  The  so-called  nitro-compounds  of  the  paraffins,  as  nitro- 
methane  and  nitro-ethane,  do  not  contain  the  group  NO2. 

4.  The  heat  of  formation  of  the  amines  indicates  that  the 
amines  of  the  fatty  series  and  those  of  the  aromatic  series 
have  not  the  same  constitution. 

5.  The  aldehydes  contain  the  group  — C — O — H,  and  are 
represented  by  the  general  formula  R — C — O — H,  being 

ii 

un  saturated. 

It  will  be  seen  that  there  is  a  general  lack  of  harmony 
between  the  results  obtained  by  thermo-chemical  methods 
and  those  obtained  by  strictly  chemical  methods.  Of  course, 
the  difficulty  is  simply  in  the  interpretation  of  the  facts. 
The  facts  themselves  cannot  be  disturbed. 

Magnetic  Rotary  Polarization  in  Relation  to  Chemical 
Constitution. — The  fact  that  when  polarized  light  passes 
through  a  substance  placed  in  a  magnetic  field  its  plane  is 


PHYSICAL  METHODS.  283 

rotated  was  discovered  by  Faraday.  W.  H.  Perkin*  has 
shown  that  there  is  a  definite  relationship  between  the 
magnetic  rotary  power  of  substances  and  their  chemical 
constitution.  The  effect  upon  polarized  light  in  a  magnetic 
field  produced  by  length  of  columns  of  liquids  related  to 
each  other  in  proportion  to  their  molecular  weights  was 
measured,  and  this  effect  compared  with  the  effect  produced 
by  water;  thus  figures  were  obtained  that  could  be  com- 
pared with  one  another.  The  result  for  each  case  is  repre- 
sented by  the  simple  formula  -  —, — ,  in  which  r  is  the 

rotation  observed,  Mw  the  molecular  weight,  and  d  the 
specific  gravity.  Dividing  this  value  in  the  case  of  any 
given  substance  by  that  obtained  in  the  case  of  water,  the 
figure  called  the  "  molecular  coefficient  of  magnetic  rota- 
tion," or  the  "  molecular  rotary  power,"  is  obtained. 

Among  the  results  thus  reached  bearing  upon  the  ques- 
tion ot  chemical  constitution  the  following  may  be  men- 
tioned : — 

The  effect  of  an  addition  of  CH2  to  a  compound  is  to 
increase  its  molecular  magnetic  rotation  by  1.023.  With 
this  result  it  is  possible  to  calculate  the  molecular  rotation 
of  any  member  of  an  homologous  series,  if  that  of  one 
member  is  known.  Take  oananthylic  acid,  C7H14O2,  as  an 
example.  Subtract  7  X  1.023  from  its  molecular  rotation, 
and  there  is  left  a  residue  in  excess  of  that  which  the  pro- 
duct would  give  if  it  consisted  of  CH2  only,  thus : — 

Molecular  rotation  of  cenanthylic  acid    .         .     7.552 
1.023x7  .  ...     7.161 


Difference 0.391 

This  residual  number  is  called  the  series  constant,  and  is 
designated  by  8.  Thus  a  formula  for  the  acids  of  this  series 
may  be  constructed :  CnH2nO2  =  0.391  -f  n(1.023). 

Similarly,  formulas  for  the  magnetic  rotary  power  of  other 
classes  of  compounds  can  be  constructed,  as  for  ethereal 
salts,  paraffins,  aldehydes,  etc. 

The  normal  and  isomeric  compounds  of  a  series  give 
different  figures.  The  isoparaffins  may  be  regarded  as 

*  For  details  see  Perkin' s  Memoir  in  the  Journal  of  the  Chemical 
Society  (London),  1884,  p.  421. 


284    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

normal  paraffins  in  which  an  atom  of  hydrogen  in  the  CH2 
group  already  associated  with  methyl  has  been  displaced 
by  methyl,  thus  : — 

CH3.CH2.CH2.CH3.  (CH3)2CH.CH2.CH3. 

Butane.  Methyl-butane  or  isopentane. 

CH3.CH2.CH2.CH2.CH3.        (CH3)2CH.CH2.CH2.CH3. 

Pentane.  Methyl-pentane  or  isohexane. 

The  influence  of  the  introduction  of  methyl  on  the  molec- 
ular rotation  is  very  marked,  as  it  increases  the  rotation 
to  a  greater  extent  than  would  result  from  the  change  in 
composition  caused  by  the  introduction  of  CH2. 

Again,  it  was  found  that  the  introduction  of  carboxyl 
causes  a  reduction  in  the  magnetic  rotation  of  alcohol  radi- 
cals when  associated  with  it,  although  the  rotation  of  the 
group  itself  is  evidently  high. 

As  regards  the  effect  of  hydroxyl,  it  was  found  that, 
both  in  the  series  of  the  paraffins  and  the  aldehydes,  the 
introduction  of  hydroxyl  in  place  of  hydrogen  increases 
the  molecular  rotation. 

On  comparing  the  molecular  rotation  of  a  paraffin  with 
that  of  the  corresponding  aldehyde  and  acid,  it  was  found 
that  oxygen,  when  displacing  two  hydrogens  on  the  same 
carbon,  has  nearly  double  the  influence  that  it  has  when 
simultaneously  combined  with  hydrogen  and  carbon. 

Unsaturated  compounds  show  a  larger  molecular  rota- 
tion than  the  corresponding  saturated  compounds. 

It  will  be  seen  from  the  above  that,  by  making  observa- 
tions on  the  magnetic  rotation  of  substances,  it  is  possible 
to  draw  certain  conclusions  regarding  their  chemical  con- 
stitution. 

The  Shape  of  Molecules. — The  methods  of  study  thus  far 
discussed  do  not  touch  the  question  as  to  the  shape  of  mole- 
cules and  the  arrangement  of  the  atoms  in  space.  In  the 
common  language  of  chemistry  we  speak  of  chain-shaped 
molecules,  of  chains  with  branches,  of  rings  and  double 
rings,  and  we  make  constant  use  of  the  hexagon  in  repre- 
senting the  structure  of  benzene  and  its  derivatives.  But, 
as  has  been  repeatedly  stated,  these  expressions  refer  to  the 
formulas,  and  the  formulas  refer  to  facts  that  have  no 
necessary  connection  with  space -relations.  While  we  repre- 
sent the  symmetry  of  benzene  by  means  of  a  circle  or  a 


PHYSICAL  *WPQM-nirt^        285 


regular  hexagon,  we  know  that  the  atoms  in  the  molecule 
of  benzene  cannot  be  arranged  in  a  plane.  But,  as  we 
know  nothing,  or  very  little,  concerning  the  arrangement 
in  space,  we  wisely  refrain  from  attempting  to  express  any- 
thing concerning  it  in  our  formulas.  When  the  physicist 

says  let  the  line  A—  B  represent  a  force,  he  can  hardly 

be  accused  of  thinking  that  the  line  is  a  picture  of  the  force. 
He  knows  that  he  represents  certain  properties  of  the  force, 
viz.,  magnitude  and  direction,  in  regard  to  which  he  has 
knowledge,  and  the  use  of  this  sign  is  of  great  assistance  in 
enabling  him  to  deal  with  a  certain  order  of  facts.  So,  too, 
the  chemist  can  fairly  represent  the  facts  with  which  he 
has  to  deal  by  means  of  symbols,  and,  if  rightly  used,  they 
are  of  great  assistance  to  him.  Still,  though  the  considera- 
tion of  space-relations  may  be  postponed  because  of  a  lack 
of  facts  upon  which  to  base  a  plausible  hypothesis,  the  prob- 
lem nevertheless  remains  to  be  solved.  Some  efforts  have 
been  made  to  reach  conclusions  regarding  the  subject,  and  of 
late  these  have  produced  a  marked  effect  upon  the  science. 

One  of  the  results  reached  may  be  stated  thus:  It  is 
shown  that  the  boiling-points,  densities,  and  indices  of 
refraction  vary  in  the  same  way ;  that  for  isomeric  com- 
pounds the  constants  of  that  one  are  largest  which  consist 
of  an  uninterrupted  chain  of  hydrocarbon  residues,  and 
that  the  constants  become  smaller  the  more  the  structure 
of  the  molecule  is  branched  and  deviates  from  one  direc- 
tion. The  data  thus  far  established  seem  to  show  that  the 
shorter  the  molecule  of  isomeric  compounds — i.  e.,  the  more 
they  approach  the  spherical  form — the  larger  is  the  molec- 
ular volume.* 

The  words  "  shorter,"  "  branched,"  etc.,  used  in  the  above 
statements,  have  primarily,  of  course,  reference  to  the  ap- 
pearance of  the  formulas  in  common  use.  Assuming  that 
these  formulas  do  in  a  crude  way  represent  the  actual  shapes 
of  the  molecules,  a  direct  connection  probably  exists  be- 
tween the  variations  in  the  physical  constants  of  isomeric 
compounds  and  the  actual  shape  of  the  molecules. 

Thus,  the  specific  gravity  of  compounds  with  long  mole- 
cules would  necessarily  be  greater  than  that  of  compounds 
with  branched  or  spherical  molecules,  for  the  same  reason 
that  we  can  get  more  rods  in  a  given  space  than  spheres  of 

*  Briihl,  Annalen  der  Chemie,  vol.  203,  p.  363. 
13* 


286    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

the  same  weight.  So,  also,  with  reference  to  the  boiling- 
point.  The  rod-shaped  molecules  offer  the  most  points  of 
contact,  the  spherical  the  least.  The  adhesion  between 
molecules  of  the  first  kind  will  hence  be  the  greatest,  and 
the  conversion  of  the  substances  into  vapor  will  be  more 
difficult  than  is  the  case  with  other  substances,  or  the  boil- 
ing-points will  be  higher.  Similar  considerations  indicate 
that  the  facts  observed  in  connection  with  the  time  of  trans- 
piration of  vapor  are  in  harmony  with  the  view  that  the 
usual  formulas  represent  to  some  extent  the  general  shape 
of  the  molecules. 

Stereochemistry. — The  most  important  speculations  bear- 
ing upon  the  question  of  the  space  relations  of  atoms  are 
based  upon  the  existence  of  such  cases  of  isomerism  as 
those  exhibited  by  the  lactic  and  tartaric  acids  and  other 
similar  compounds. 

A  study  of  the  effects  produced  upon  polarized  light 
when  it  passes  through  certain  chemical  compounds  led 
Le  Bel  and  Van't  Hoff  to  propose  an  hypothesis  in  regard 
to  the  arrangement  of  atoms  in  the  molecules  of  these 
compounds.  Reference  has  already  been  made  to  this 
hypothesis  (see  ante,  p.  220),  to  account  for  some  cases  of 
isomerism  which  cannot  otherwise  be  explained.  Thus 
there  are  three  lactic  acids,  which,  according  to  all  we  know 
regarding  their  methods  of  formation  and  their  chemical 
transformations,  must  be  represented  by  the  same  ordinary 


.,4- 


chemical   formula   GIL— C— COOH.     While   these  sub- 


stances  are  very  similar  in  all  other  respects,  they  differ 
in  their  effects  upon  polarized  light.  One,  ordinary  lactic 
acid,  has  no  perceptible  effect  upon  polarized  light ;  another, 
paralactic  acid,  turns  the  plane  of  polarization  to  the  right  ; 
while  the  third  turns  it  to  the  left.  Further,  there  are 
three  acids,  dextro  tartaric  and  Isevo- tartaric  acids,  and 
racemic  acid,  all  of  which  appear  to  have  the  same  struc- 
ture, as  far  as  our  formulas  express  structure.  One  of 
these  turns  the  plane  of  polarization  to  the  right,  another 
turns  it  to  the  left,  while  racemic  acid  is  optically  inactive. 
These  facts  can  be  explained  according  to  the  hypothesis 


STEREOCHEMISTR  Y. 


28T 


as  follows :  Let  it  be  supposed  that  in  a  carbon  compound 
one  carbon  atom  is  situated  at  the  centre  of  a  tetrahedron, 
and  that  the  four  atoms  or  groups  which  it  holds  in  com- 
bination are  at  the  angles  of  the  tetrahedron,  as  repre- 
sented in  Fig.  1.  If  all  these  groups  are  different  in  kind, 
and  only  in  this  case,  it  is  possible  to  arrange  them  in  two 
ways  with  reference  to  the  carbon  atom.  The  difference 
between  the  two  arrangements  is  that  which  is  observed 
between  either  one  and  its  reflection  in  a  mirror.  Imper- 
fectly the  second  arrangement  of  tne  figure  is  represented 
in  Fig.  2. 


FIG.  1. 


FIG.  2. 


R. 


A  carbon  atom  in  combination  with  four  different  kinds 
of  atoms  or  groups  is  called  an  asymmetrical  carbon  atom. 
Whenever,  therefore,  a  compound  contains  an  asymmet- 
rical carbon  atom,  there  are  two  possible  arrangements  of 
its  parts  in  space  which  correspond  to  the  complementary 
tetrahedrons,  viz.,  the  right-handed  and  the  left-handed 
tetrahedron.  The  third,  optically  inactive  modification  of 
a  substance  is  believed  to  be  a  compound  of  the  other 
two.  In  the  lactic  acids  and  in  the  tartaric  acids  the 
asymmetrical  carbon  atom  is  present,  as  is  shown  by  the 
formulas : — 

H  H 


-C 


CIL-C—  COOH 


>H 

Lactic  acid. 


HO- C— COOH 
HO-CH.COOH 

Tartaric  acid. 


In  lactic  acid  the  central  carbon  is  in  combination  with 
1)OH  ;  2)COOH ;  3)H ;  and  4)CH3.     In  tartaric  acid  the 


288    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

four   different   groups   and    atoms   are   1)H;    2)COOH; 
3)CH.COOH ;  and  4)OH. 

Our  knowledge  on  this  subject  at  present  may  be  fairly 
summed  up  in  the  following  sentences : — 

1.  Optically  active  substances  always   contain   one  or 
more  asymmetrical  carbon  atoms. 

2.  Substances  which  do  not  contain  asymmetrical  -carbon 
atoms  have  no  rotary  power. 

3.  Isomerism  is  possible  in  the  case  of  any  compound 
containing  an  asymmetrical  carbon  atom. 

According  to  this  hypothesis,  the  affinities  of  an  atom 
are  exerted  in  definite  directions,  i.  e.,  toward  the  angles 
of  a  tetrahedron  at  the  centre  of  which  the  carbon  atom  is 
supposed  to  be.  Von  Baeyer  has  attempted  to  show  that, 
on  this  assumption,  the  fact  that  rings  of  six  carbon  atoms 
are  easily  formed  and  are  stable  finds  a  plausible  explana- 
tion. This  will  be  easily  understood  if  we  conceive  the 
affinities  to  be  lines  extending  from  the  carbon  atoms  in  the 
directions  indicated.  When  six  carbon  atoms  are  brought 
together  they  can  combine  if  the  directions  of  their  lines 
are  changed  only  very  slightly ;  while,  in  the  case  of  four 
or  five  carbon  atoms,  the  displacement  would  be  consider- 
able, and,  as  Von  Baeyer  expresses  it,  the  tension  would  be 
so  great  that  the  compound  would  be  unstable.  What 
value  to  attach  to  such  considerations  at  present  it  is  diffi- 
cult to  say.  In  this,  as  in  so  many  other  cases  in  which 
our  knowledge  is  imperfect,  we  can  only  wait  until  further 
work  shall  have  furnished  us  with  more  facts. 

That  branch  of  chemistry  which  has  to  deal  with  the 
phenomena  of  space-relations  is  known  as  stereochemistry. 
Within  the  last  few  years  there  has  been  great  activity  in 
this  branch,  and  it  seems  highly  probable  that  more  and 
more  attention  will  be  given  to  it,  as  the  results  already 
reached  are  of  great  interest. 

The  beautiful  work  of  E.  Fischer  on  the  sugars  has  fur- 
nished a  large  amount  of  evidence  in  favor  of  the  theory 
of  the  asymmetrical  carbon  atom.  Through  this  work 
many  compounds  have  been  brought  to  light  the  existence 
of  which  it  is  impossible  to  account  for  by  the  conception 
of  atomic  linkage  alone,  though  they  are  readily  explained 
by  the  aid  of  the  Le  Bel-Van't  Hoff  hypothesis. 

There  is  another  kind  of  isomerism  met  with  among 
compounds  of  carbon  that  appears  also  to  be  due  to  space- 


STEREOCHEMISTR  Y. 


289 


relations.  This  is  best  illustrated  by  the  isomerism  of 
raaleic  and  furaaric  acids.  Both  these  apparently  have 

CH.COOH 
the  constitution  represented  by  the  formula   ||  ; 

CH.COOH 

that  is  to  say,  both  appear  to  be  symmetrical  dicarboxyl 
derivatives  of  ethylene.  As  will  be  seen,  there  is  no  asym- 
metrical carbon  atom  contained  in  these  compounds,  and 
therefore  this  case  of  isomerism  cannot  be  explained  in  the 
same  way  as  that  of  the  three  lactic  acids.  An  explana- 
tion can,  however,  be  offered,  based  upon  stereochemical 
conceptions.  Taking  the  same  view  of  the  carbon  atom  as 
that  already  presented,  a  compound  containing  two  carbon 
atoms  united  in  the  simplest  way  should  be  represented  by 
this  stereochemical  figure : — 


The  two  carbon  atoms  are  united  by  one  bond  each  at 
O,  while  the  one  represented  by  the  upper  part  of  the  figure 
is  in  combination  with  X,  Y,  and  Z,  and  the  one  repre- 
sented by  the  lower  part  of  the  figure  is  in  combination 
with  A,  B,  and  C.  Now,  as  we  may  assume  that  the  two 
carbon  atoms  can  rotate  freely  around  the  axis  formed  by 
the  union  of  the  two  bonds  at  O,  they  will  arrange  them- 
selves in  accordance  with  the  specific  affinities  of  the  groups 
or  atoms  with  which  they  are  in  combination,  and  isomer- 
ism is  possible  only  when  one  or  the  other  of  the  carbon 
atoms  is  asymmetrical. 

Passing  now  to  compounds  containing  carbon  atoms 
doubly  linked,  formulas  1  and  2  (page  290)  represent  this 


290    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

condition  stereochemically.    Interpreting  these  figures  in  as 
simple  a  way  as  possible,  we  must  conclude  that  the  two 


COOH  H^T-         —  ^COOH 


COOH        COOH 


carbon  atoms  represented  as  in  combination  cannot  rotate 
independently  of  each  other,  and  we  can  therefore  conceive 

CHX 
of  two  isomeric  compounds  of  the  general  formula  II 

CHX 

One  of  these  is  represented  by  formula  1,  in  which  X  is 
carboxyl,  COOH  ;  the  other  is  represented  by  formula  2. 
In  one  the  two  carboxyl  groups  are  on  the  same  side  of  the 
molecule  ;  in  the  other  they  are  on  different  sides.  One  of 
the  compounds  thus  represented  is  believed  to  be  maleic 
acid,  the  other  fumaric  acid;  and  as  maleic  acid  readily 
loses  the  elements  of  water  and  forms  an  anhydride,  while 
fumaric  acid  does  not,  formula  1,  in  which  the  two  car- 
boxyl groups  are  represented  as  being  on  the  same  side  of 
the  molecule,  is  assigned  to  maleic  acid. 

This  view  in  regard  to  the  isomerism  of  these  two  acids 
is  now  pretty  generally  accepted.  There  are  several  other 
examples  of  this  kind  of  isomerism  known. 

Stereoisomerism  due  to  Nitrogen.  —  Still  another  kind  of 
isomerism  inexplicable  by  the  aid  of  the  hypothesis  of 
atomic  linking  alone  is  presented  by  certain  compounds  of 
nitrogen.  It  was  first  observed  in  the  case  of  the  oximes, 
and  these  furnish  the  best  examples.  The  oximes  are 
formed  by  the  action  of  hydroxylamine  on  either  alde- 
hydes (aldoximes)  or  acetones  (acetoximes),  the  following 
equations  representing  the  reactions  :  — 


CO  +  H?NOH  =          )C=:KOH  -f-  H2O. 
H./  H/ 

Aldehyde.  Aldoxime, 


STEREOCHEMISTRY.  291 

CH3X  CH 

^CO  +  H2NOH  =  )C=N.OH  +  H2O. 

CH/  CH/ 

Acetone.  Acetoxime. 

In  the  case  of  aldehydes  and  of  some  acetones  containing 
residues  of  aromatic  hydrocarbons  a  kind  of  isomerism 
presents  itself  which  seems  to  be  like  that  existing  be- 
tween maleic  and  fumaric  acids.  Thus  benzoic  aldehyde, 
C6H5.CHO,  yields  two  oximes,  C6H5.CH.N.OH.  Both  have 
been  shown  to  have  the  structure  represented  by  the  for- 

C6H 
mula  /C=N.OH.      It    has     been    suggested    by 

H7 

Hantzsch  and  Werner  that  this  is  a  case  of  stereoisomerism 
due,  in  the  first  instance,  to  the  relations  between  the  nitro- 
gen atom  and  the  carbon  atom  with  which  it  is  combined. 
Their  idea  is  that  nitrogen  is  situated  as  at  one  of  the  solid 
angles  of  a  tetrahedron  and  that  its  bonds  extend  toward  the 
other  three  solid  angles  as  represented  in  the  figure  below  : — 


Now,  if  the  nitrogen  is  in  combination  with  carbon  by 
two  bonds,  then  there  are  two  ways  in  which  the  third  bond 
may  be  directed,  as  indicated  by  these  formulas,  which  arc, 
of  course,  to  be  interpreted  stereochemically  : — 

X— C— Y  X— C— Y 

II  and  || 

N— A  A— N 

If  X  and  Y  are  different,  as  is  the  case  in  benzaldoxime, 
C6H5 — C — H,  these  two  arrangements  represent  different 

N— OH 

compounds : — 

C6H5-C— H  C6H5— C-H 

N— OH  HO— N 


292    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

Whereas,  if  X  and  Y  are  the  same,  the  two  arrange- 
ments do  not  represent  different  compounds.  Thus,  there 
should  be  but  one  oxime  derivable  from  benzophenone, 
C6H5— CO— C6H5.  For  the  two  formulas 

C6H5-C-C6H5  C6H-C-C6H5 

II  and  i| 

N— OH  HO— N 

are  identical. 

It  has  been  found  that  aromatic  aldehydes  and  unsym- 
metrical  aromatic  acetones  yield  isomeric  oximes. 

There  is,  further,  some  reason  to  believe  that  diazo-coin- 
pounds  can  exist  in  two  stereoisomeric  forms  corresponding 
to  the  two  formulas:  — 

C6H5— N  C6H—  N 

II  and  ||  . 

N— X  X— N 


THE  STUDY  OF  CHEMICAL  AFFINITY.         293 


CHAPTER    XVIII. 

THE   STUDY  OF   CHEMICAL   AFFINITY. 

Introduction. — Our  study  thus  far  has  been  confined  to 
questions  of  composition  and  constitution,  and,  indeed,  as 
already  stated,  the  main  object  of  this  book  is  to  discuss 
the  facts  upon  which  the  prevailing  views  regarding  con- 
stitution are  based.  One  cannot,  indeed,  overestimate  the 
value  of  the  work  that  has  been  done  in  the  way  of  in- 
vestigating chemical  constitution.  The  widespread  applica- 
bility of  the  prevailing  views  to  the  interpretation  of 
chemical  phenomena,  the  brilliant  successes  that  have  been 
achieved  in  the  building  up  of  complex  compounds  by 
methods  suggested  by  these  views,  furnish  overwhelming 
evidence  of  the  truth  there  is  in  them.  A  complete  study 
of  chemistry,  however,  involves  not  only  the  subject  of 
constitution,  but  that  of  the  laws  governing  chemical 
action  itself.  We  should  not  be  satisfied  with  studying 
chemical  compounds  after  they  are  formed,  but  we  should, 
if  possible,  make  observations  during  the  act  of  forma- 
tion, and  thus  get  as  much  insight  as  possible  into  the 
nature  of  the  act.  For  various  reasons  there  are  serious 
difficulties  in  the  way  of  such  observations,  and,  although 
a  great  deal  of  work  of  the  kind  indicated  has  been  done, 
the  results  up  to  quite  recently  have  not  been  of  special 
importance. 

The  Nature  of  the  Problem. — We  know  that  when  hy- 
drogen and  chlorine  are  brought  together  they  combine 
with  evolution  of  great  heat.  The  act  is,  in  most  cases, 
instantaneous,  and  it  is  therefore  practically  impossible  to 
make  any  observations  during  its  progress.  In  the  smallest 
particle  of  the  resulting  gas  both  hydrogen  and  chlorine 
are  found  in  definite  proportions ;  and  iii  order  to  separate 
them,  or  to  decompose  the  compound,  an  expenditure  of 
energy  is  necessary.  The  phenomenon  suggests  such  phe- 
nomena of  attraction  as  those  of  gravitation,  electricity, 


294    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

and  magnetism.  We,  therefore,  commonly  picture  to  our- 
selves the  atoms  of  hydrogen  and  chlorine  as  drawn  to- 
gether by  an  attractive  force,  much  as  the  stone  thrown 
upward  is  drawri  to  the  earth  or  as  the  electrified  body  is 
attracted  by  other  bodies.  In  each  case,  in  order  to  sepa- 
rate the  united  substances  an  expenditure  of  energy  is 
necessary.  Without  any  conception  in  regard  to  the  nature 
of  this  chemical  force  we  may  for  convenience  call  it  chem- 
ical affinity.  The  first  object  in  view  in  the  study  of  this 
force  is  to  measure  its  intensity  in  different  cases.  What 
is  the  difference  between  the  power  of  the  action  exerted 
between  hydrogen  and  chlorine  and  that  exerted  between 
hydrogen  and  oxygen,  etc.,  etc.?  Is  it  dependent  upon 
the  nature  of  the  substance,  or  is  it  independent  of  the 
nature  and  only  dependent  upon  the  mass?  Such  are 
some  of  the  questions  that  suggest  themselves.  What  we 
want  is  a  measure  of  this  power  exerted  between  the  atoms 
of  all  the  elements  under  different  conditions.  It  is  plain 
that  with  this  knowledge  we  should  be  in  a  position  to 
predict  in  every  case  the  exact  character  of  the  reaction 
or  reactions  between  any  substances.  Let  us  see  what 
progress  has  been  made  in  this  line. 

Rough  Measurements  of  Affinities. — It  has  long  been 
known  that  some  elements  combine  with  ease  and  form 
stable  compounds,  while  others  combine  with  difficulty 
and  form  unstable  compounds,  and  others  still  do  not  com- 
bine at  all,  and  it  has  been  said  that  the  elements  of  the 
first  class  have  a  strong  affinity  for  one  another,  those  of 
the  second  class  have  a  weak  affinity  for  one  another,  and 
those  of  the  third  class  have  no  affinity  for  one  another. 
Thus  hydrogen  and  oxgen,  phosphorus  and  oxygen,  potas- 
sium and  oxygen,  etc.,  etc.,  have  a  strong  affinity  for  each 
other;  nitrogen  and  hydrogen,  carbon  and  hydrogen, 
chlorine  and  iodine,  etc.,  etc.,  have  a  weak  affinity  for 
each  other;  and  fluorine  and  oxygen,  chromium  and  hy- 
drogen, etc.,  etc.,  have  no  affinity  for  each  other.  Further, 
in  certain  natural  groups  of  elements  a  gradation  in  affinity 
in  accordance  with  the  gradations  in  the  atomic  weights 
is  easily  recognized,  as,  for  example,  in  the  group  of  the 
so-called  halogens  chlorine  is  said  to  be  a  stronger  element 
than  bromine,  and  bromine  a  stronger  element  than  iodine. 
These  statements  are  based  upon  observations  on  the  action 


THE  STUDY  OF  CHEMICAL  AFFINITY.         295 

of  chlorine  on  compounds  of  bromine  and  iodine.  As 
chlorine  decomposes  the  compounds  of  these  two  elements, 
it  is  said  to  be  a  stronger  element,  by  which  is  meant  that 
it  has  a  stronger  affinity  towards  some  other  elements. 
Hosts  of  similar  observations  have  been  made,  and  it  is 
generally  recognized  that  chemical  reactions  take  place  in 
consequence  of  differences  in  the  affinities  of  the  elements 
for  one  another.  If,  for  example,  the  elements  A  and  B 
have  for  each  other  the  same  affinity  as  C  and  D,  and  this 
is  the  same  as  the  affinity  of  A  for  D,  A  for  C,  B  for  C, 
and  B  for  D,  then,  on  bringing  the  compounds  AB  and 
CD  together,  no  change  takes  place;  but  if  the  affinity  of 
A  for  C  is  greater  than  that  of  A  for  B,  and  that  of  C 
for  D  greater  than  that  of  B  for  C,  then  the  change  repre- 
sented in  the  equation 

AB    +     CD    =    AC    +     BD 

will  take  place. 

By  careful  observations  of  chemical  reactions,  then,  it  is 
possible  to  get  some  idea  regarding  the  relative  strength  of 
the  affinities  of  the  elements,  but  much  more  refined  methods 
are  necessary. 

Disturbing  Influences. — A  serious  difficulty  in  the  way 
of  measuring  affinity  by  chemical  observations  is  found  in 
the  fact  that  decompositions  do  not  always  take  place  in 
accordance  simply  with  the  strength  of  the  affinities  of  the 
elements  which  take  part  in  the  reaction ;  and,  indeed,  this 
is  markedly  true  of  those  reactions  which  we  most  fre- 
quently have  to  deal  with.  The  most  common  disturbing 
causes  are  changes  in  the  state  of  aggregation  of  the  forms 
of  matter.  Thus,  when  by  adding  a  substance  A  to  a  sub- 
stance B,  a  substance  C,  which  is  a  gas  at  the  temperature 
employed,  can  be  formed,  then  in  general  the  gas  will  be 
formed,  each  particle  escaping  as  soon  as  formed,  and  thus 
-being  removed  from  the  sphere  of  action.  An  example  of 
this  kind  is  furnished  by  the  decomposition  of  a  chloride 
or  a  nitrate  by  sulphuric  acid.  In  each  case  with  the  aid 
of  slight  elevation  of  temperature  there  is  decomposition, 
and  finally  there  is  complete  displacement  of  the  hydro- 
chloric or  nitric  acid  by  sulphuric  acid.  From  this  the 
conclusion  was  formerly  drawn  that  sulphuric  acid  is  a 
stronger  acid  than  the  other  two.  But  all  methods  of  com- 


296     PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

paring  these  acids  under  the  same  conditions  show  that 
hydrochloric  and  nitric  acids  are,  as  a  matter  of  fact,  much 
stronger  acids  than  sulphuric  acid,  and  the  cause  of  de- 
composition of  the  nitrates  and  chlorides  by  sulphuric  acid 
is  to  be  found  in  the  fact  that  volatile  products  are  formed. 
Again,  when  a  product  of  the  reaction  of  two  substances 
in  solution  is  an  insoluble  solid,  the  decomposition  will 
generally  take  place  and  be  complete,  independently  of 
the  strength  of  the  affinities.  The  affinity  of  chlorine  for 
sodium  is  greater  than  that  of  chlorine  for  lead ;  but,  if  to 
a  solution  of  sodium  chloride  a  solution  of  lead  acetate  is 
added,  lead  chloride  ia  thrown  down,  though  not  com- 
pletely. Here,  as  in  the  case  of  the  formation  of  a  volatile 
product,  the  product  is  removed  from  the  sphere  of  action 
as  soon  as  formed,  and  this  causes  an  entire  change  of  the 
process. 

From  these  examples  it  will  be  clear  that  the  simple  fact 
that  one  substance,  A,  decomposes  another,  BG,  and  unites 
with  C  to  form  A  C,  is  not  sufficient  evidence  that  the 
affinity  of  A  for  C  is  greater  than  that  of  B  for  (7.  This 
is  especially  the  case  when  the  product,  A  C,  is  a  gas  or  an 
insoluble  solid. 

Attempts  to  measure  Affinity  by  Observations  on  the  Heat 
evolved  in  Chemical  Reactions. — If  a  mass  of  hydrogen  and 
a  mass  of  chlorine  consisted  of  isolated  atoms  at  rest,  and, 
after  the  combination,  the  molecules  as  well  as  their  con- 
stituent atoms  were  at  rest,  then  the  heat  evolved  in  the 
act  of  combination  would  probably  be  the  result  of  the 
transformation  of  the  potential  energy  of  the  atoms  into 
kinetic  energy,  and  it  would  be  a  measure  of  the  affinity 
exerted  between  the  atoms,  But  not  one  of  these  condi- 
tions can  be  assumed  with  any  confidence,  and  most  of  them 
are  undoubtedly  not  true.  We  have  abundant  evidence  to 
show  that  the  mass  of  hydrogen  and  that  of  chlorine  do 
not  consist  of  isolated  atoms.  Taking  then  the  reaction 
between  hydrogen  and  chlorine,  it  is  clear  that  it  is  not 
simply  a  combination  of  atoms,  but  that  the  act  of  com- 
bination between  the  atoms  must  bo  preceded  by  the  de- 
composition of  the  molecules  of  hydrogen  and  those  of 
chlorine.  According  to  our  present  views,  the  reactions 
must  be  represented  in  this  way  : — 


THE  STUDY  OF  CHEMICAL  AFFINITY.         297 

(1)  H2  +  CI,  =  H  +  H  +  C1  +  C1; 

(2)  H  +  H-f  Cl-f  C1  =  HC1  +  HC1. 

The  heat  evolved  in  the  reaction  is  therefore  not  simply 
the  result  of  the  combination  of  hydrogen  and  chlorine,  but 
it  is  this  heat  less  that  which  is  used  in  decomposing  the 
molecules  of  hydrogen  and  of  chlorine  into  atoms.  The 
heat  measured  is  the  difference  between  two  quantities; 
and  we  have  no  way  of  estimating  the  value  of  these  quan- 
tities. This  is  true  of  every  chemical  reaction.  The  heat 
evolved  or  absorbed  in  the  reaction  is  the  difference  between 
two  or  more  quantities,  and  it  is  not,  therefore,  a  measure 
of  affinity. 

Nevertheless,  some  knowledge  regarding  the  relations 
which  the  affinities  of  elements  bear  to  one  another  can  be 
gained  by  a  study  of  the  heat  evolved  in  their  reactions. 
Thus,  the  following  results  have  been  obtained  in  the  study 
of  chlorine,  bromine,  and  iodine  : — 

[H2,C12]  =  2[H,C1]  —  [H,H]  —  [C1,C1]  —  44,000  c. 
[H2,Br2]  =  2[H,Br]  —  [H,H]  —  [Br,Br]  ===  16,880  c. 
[H,,IJ  =2[H,I]  -  [H,H]  -  [1,1]  -  12,072  c. 

The  figures  thus  obtained  are  not  proportional  to  the 
affinities  of  chlorine,  bromine,  and  iodine  for  hydrogen, 
but,  nevertheless,  the  affinities  in  all  probability  vary  in 
the  same  order. 

The  difficulties  are  much  increased  in  more  complicated 
cases,  and  it  will  therefore  be  seen  that  it  is  impossible  to 
measure  the  affinity  between  atoms  by  means  of  the  heat 
evolved  in  reactions. 

Resultant  Affinity. — While  affinity  in  the  strictest  sense 
must  be  regarded  as  a  simple  force  acting  between  atoms, 
yet  we  may  speak  of  the  affinity  of  one  molecule  for 
another,  meaning  by  the  expression  the  resultant  of  the 
various  atomic  affinities  which  are  brought  into  play  when 
the  two  molecules  combine.  This  resultant  affinity  is  of 
course  a  complex  quantity,  as  has  been  pointed  out.  Never- 
theless, it  is  desirable  to  make  as  many  measurements  of  it 
as  possible.  The  meaning  of  the  term  will  be  clear  by  a 
reference  to  the  cases  of  chlorine,  bromine,  and  iodine. 
The  measurements  recorded  in  the  last  paragraph  represent 
the  resultant  affinities  of  these  elements  for  hydrogen  in 


298    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

terras  of  heat  units.  In  a  similar  way  we  may  speak  of  the 
resultant  affinities  of  acids  and  bases  for  one  another.  Thus 
the  resultant  affinity  of  hydrochloric  acid  towards  caustic 
soda  involves  the  following  reactions  :  — 

(1)  H—  Cl+Na—  0-H    ==  H-f  Cl+Na+(O—  H). 

(2)  H+Cl+Na+(0—  H)=  Na—  Cl+H-O—  H. 

Notwithstanding  the  complex  character  of  measurements 
made  in  such  cases  they  are  of  service  in  dealing  with 
chemical  questions. 

Heat  of  Neutralization.  —  Avidity  of  Acids.  —  Among  the 
measurements  that  have  proved  of  value  in  connection  with 
the  study  of  the  general  problem  of  affinity  are  those  fur- 
nished by  the  heat  of  neutralization  of  acids  and  bases. 
This  subject  has  been  investigated  very  extensively  by 
Thomsen.  The  general  method  of  work  consisted  in  de- 
termining the  heat  evolved  when  equivalent  quantities 
of  different  acids  are  neutralized  by  the  same  base,  and 
equivalent  quantities  of  different  bases  are  neutralized  by 
the  same  acid.  Knowing  the  heat  evolved  in  the  reactions 
between  the  various  acids  and  bases,  it  is  possible  to  learn 
something  in  regard  to  what  takes  place  when  acids  act 
upon  salts  in  those  cases  in  which  decomposition  is  not 
evident.  Thus,  when  nitric  acid  acts  upon  sodium  sulphate 
in  solution,  several  changes  are  possible,  as  represented  in 
the  equations  :  — 

(1)  Na2SO,+    HNO3=NaHSO4  +    NaN03; 

(2)  Na2S04  +  2HNO3  =  H2S04      +  2NaN03  ; 

(3)  2Na2SO,  -f  4HN03  = 

4  -f  2NaN03  +  H2SO4  -f  2HNO3. 


As  all  the  substances  involved  in  these  reactions  are 
soluble  in  water,  and  the  reactions  are  studied  in  water 
solution,  it  is  clear  that  by  ordinary  methods  it  would  be 
impossible  to  tell  which  of  them  take  place.  By  thermo- 
chemical  methods,  however,  it  has  been  shown  that  in  this 
and  in  all  similar  cases  the  base  is  divided  between  the 
two  acids  and  generally  more  goes  to  one  acid  than  to  the 
other.  Further,  it  is  possible  to  measure  the  division  of 
the  base  between  the  acids,  and  in  this  way  measurements 
of  the  relative  strengths  of  the  acids  are  obtained.  The 


THE  STUDY  OF  CHEMICAL  AFFINITY.        299 

figures  representing  the  strengths  of  the  acids  measured  in. 
this  way  are  called  by  Thomsen  the  avidities  of  the  acids. 
In  the  case  taken  as  an  illustration  it  was  found  that  in 
aqueous  solution  of  proper  dilution  two  thirds  of  the  soda 
combines  with  the  nitric  acid  and  one-third  with  the  sul- 
phuric acid.  Therefore,  it  appears  that  the  avidity  of  nitric 
acid,  is  twice  as  great  as  that  of  sulphuric  acid.  Of  all 
acids  investigated,  nitric  and  hydrochloric  acids  were  found 
to  have  the  greatest  avidity.  Calling  this  1.00,  the  avidi- 
ties of  some  other  acids  are  represented  in  the  following 
table :— 


Acids. 

Avidity. 

1  M 

1  00 

1     < 

'     Hydrochloric  acid 

1  00 

1     ' 

'     Hydrobromic    " 

. 

089 

1 

'     Hydriodic         "      . 

079 

i     < 

'     Sulphuric         " 

049 

I 

'     Selenic              "     . 

. 

045 

1 

'     Trichloracetic  " 

k 

036 

1 
* 

'      Orthophosphoric  acid 
'      Oxalic  acid 

• 

0.25 
024 

'     Monochloracetic  acid 

§ 

0.09 

1 

'      Hydrofluoric  acid   . 

. 

0.05 

| 

'     Tartaric  acid  . 

005 

'     Citric        '      . 

§ 

005 

i 

003 

I 

'     Boric         '      . 

I 

001 

i 

'     Silicic        '      . 

. 

0.00 

i 

'     Hydrocyanic  acid    . 

. 

0.00 

It  appears  that  the  figures  given  represent  the  numerical 
relations  between  some  common  property  possessed  by 
acids,  a  property  which  we  have  vaguely  in  mind  when 
we  speak  of  the  strength  of  acids.  This  appears  more 
clearly  when  acids  and  bases  are  studied  by  other 
methods. 

Mass- action. — At  the  beginning  of  this  century  ap- 
peared C.  L.  Berthollet's  Essai  de  statique  chimique.  The 
author  took  the  ground  that  chemical  affinity  is  essentially 
the  same  as  gravitation.  If  this  be  true,  however,  chem- 
ical action  must  be  proportional  to  the  masses  of  the 
substances  acting  upon  each  other.  This  carries  with  it 
another  conclusion,  viz.:  That  when  a  substance  AB  is 
decomposed  by  CD  with  the  formation  of  AC  and  BD, 
the  action  will  grow  less  and  less,  as  the  masses  of  the 


300    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

original  substances  decrease,  and,  finally,  a  stage  will  be 
reached  before  the  decomposition  is  complete  when  no 
further  action  will  take  place,  or  a  state  of  equilibrium 
will  be  established  between  the  four  substances.  Berthol- 
let's  fundamental  proposition  is  this  : — 

"  Every  substance  which  tends  to  enter  into  combination 
acts  in  proportion  to  its  affinity  and  its  mass." 

Not  much  attention  was  paid  to  the  work  of  Berthollet 
until  comparatively  recently.  Now,  especially  through 
the  labors  of  Guldberg  and  Waage  and  of  Ostwald,  the 
truth  of  this  fundamental  proposition  has  been  established 
and  the  study  of  affinity  has  been  materially  advanced. 

In  1852  Rose  published  a  paper  on  the  action  of  water 
on  various  chemical  compounds  and  showed  that  there  is 
some  connection  between  the  quantity  of  water  used  and 
the  extent  of  the  decomposition.  A  little  later  Bunsen 
showed  that,  when  to  a  mixture  of  hydrogen  and  carbon 
monoxide  not  enough  oxygen  is  added  to  oxidize  the  gases 
completely,  the  oxygen  is  divided  between  the  two  gases 
not  in  proportion  to  their  quantities,  but  so  that  the  quan- 
ties  of  carbon  dioxide  and  water  produced  stand  in  simple 
rational  ratio  to  each  other.  This  result  was  afterward 
shown  to  be  erroneous  by  Horstmann.  Many  observa- 
tions made  since  have  shown  that  the  extent  of  a  chemical 
reaction  is  unquestionably  influenced  by  the  mass  of  the 
substances  brought  into  action. 

The  important  papers  of  Guldberg  and  Waage  appeared 
in  1867  and  later.  The  starting-point  of  their  investiga- 
tions will  appear  from  the  following  quotation : — 

"  When  two  substances  A  and  B  are  transformed  into 
two  new  substances,  A'  and  B1 ',  the  chemical  force  with 
which  A  and  B  act  upon  each  other  is  measured  by  the 
quantity  of  the  new  substances  formed  in  unit  time." 

"The  quantity  of  a  substance  in  unit  volume  of  the 
compound  in  which  the  chemical  change  takes  place  we 
call  the  active  mass  of  the  substance." 

"  The  chemical  force  with  which  two  substances,  A  and 
B,  act  upon  each  other  is  equal  to  the  product  of  their 
active  masses,  multiplied  by  the  coefficients  of  affinity." 

"By  coefficient  of  affinity  a  coefficient  is  understood 
which  is  dependent  upon  the  chemical  nature  of  the  two 
substances  and  upon  the  temperature.  If  the  active  masses 
of  A  and  B  are  represented  by  p  and  q,  and  the  coefficient 


THE  STUDY  OF  CHEMICAL  AFFINITY.         301 

of  affinity  by  k,  then  the  force  acting  between  A  and  B  is 
expressed  by  kpq.  .  .  ." 

"  When  in  a  chemical  process  A  and  B  are  transformed 
into  A'  and  B',  and  A  and  B'  can  at  the  same  time  be 
transformed  into  A  and  B,  equilibrium  will  be  established 
when  the  force  acting  between  A  and  B  is  equal  to  the 
force  acting  between  A  and  B'" 

"  If  the  active  masses  of  A  and  B'  are  represented  by  pf 
and  q'  and  their  coefficient  of  affinity  by  kf,  the  chemical 
force  acting  between  A  and  Bf  is  expressed  by  k'p'q'." 

"  The  condition  of  equilibrium  is  therefore  expressed  by 
the  equation  kpq=k'p'q." 

The  above  statements  taken  together  form  the  law  of 
mass-action.  The  law  as  thus  stated  can  be  tested  and  has 
been  tested  by  the  authors  in  a  number  of  ways  and  found 
to  hold  true. 

Measurements  of  Coefficients  of  Affinity. — The  law  of  mass- 
action  as  stated  makes  it  possible  to  measure  what  Guld- 
berg  and  Waage  call  the  coefficients  of  affinity  in  cases 
where  reversible  reactions  of  the  kind  above  referred  to 
take  place,  or  at  least  to  measure  the  ratio  between  the 
two  coefficients  involved.  Thus,  in  the  equation 

kpq   =   k'p'q', 

p,  q,  p',  qf  represent  the  number  of  equivalents  of  the  four 
substances  which  take  part  in  a  reversible  reaction.  If  these 

are  known,  it  is  clear  that  the  ratio  p  is  also  known. 

An  example  of  the  application  of  this  method  is  furnished 
by  the  case  of  the  action  of  alcohol  on  acetic  acid  studied 
by  Berthelot  and  St.  Gilles.  When  these  substances  are 
brought  together  the  formation  of  acetic  ether  begins,  but 
after  a  time  stops.  At  first  the  reaction 

CH3.COOH  +  C2H5OH  =  CH3.COOC2H5  +  H2O 

takes  place.  But  the  substances  formed  also  act  upon  each 
other  to  some  extent  in  the  reverse  way.  It  was  found 
that  when 

acid  +  alcohol  =  \  (ether  -j-  water) 

the  action  stopped  or  equilibrium  was  established,  or,  in 

14 


302    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 


this  case,  p-  =  %.     It  was  further  found  that  by  changing 
/c 

the  proportions  of  the  substances  the  quantity  of  ether 
formed  was  changed,  and  the  quantity  formed  agreed  with 
that  calculated  by  means  of  the  equation  of  equilibrium 

when  y  =  J. 

Later  it  was  shown  that  the  results  obtained  by  Thomsen 
in  his  studies  on  neutralization  are  in  perfect  accordance 
with  the  theory  of  Guldberg  and  Waage  ;  what  Thomsen 
calls  the  avidity  of  acids  and  bases  being  the  same  as  the 
coefficient  of  affinity. 

Velocity  of  Chemical  Change  as  a  Means  of  Measuring 
Coefficients  of  Affinity.  —  The  coefficient  of  affinity  may,  ac- 
cording to  Guldberg  and  Waage,  be  measured  by  deter- 
mining the  velocity  of  chemical  changes.  Their  first 
proposition  (see  ante}  is  :  — 

"  When  two  substances,  A  and  B,  are  transformed  into 
two  new  substances,  A'  and  -B',  the  chemical  force  with 
which  A  and  B  act  upon  each  other  is  measured  by  the 
quantity  of  the  new  substances  formed  in  unit  time." 

The  force  tending  to  bring  about  the  reaction  is  repre- 
sented by  k  p  q,  and  since  the  velocity  is  proportioned  to 
the  active  force  we  have 

v  =  4>(kpq), 

in  which  0  is  the  coefficient  of  velocity.  This  equation 
holds  for  reactions  which  proceed  in  one  direction  only. 
When  the  reaction  is  reversible  the  total  velocity  will  be 
equal  to  the  difference  between  the  velocities  in  opposite 
directions,  or 


This  conception  has  been  tested  in  a  number  of  reac- 
tions. Among  them,  that  of  the  action  of  water  on  acet- 
amide  and  that  of  water  on  methyl  acetate  have  been 
studied  by  Ostwald,  with  the  result  of  showing  it  to  be 
well  founded, 

Volume-  chemical  Method.  —  Ostwald  has  tested  the  law  of 
mass  action  by  means  of  observations  on  specific  gravi- 
ties of  solutions,  and  has  in  this  way  reached  conclusions 


THE  STUDY  OF  CHEMICAL  AFFINITY.         303 

regarding  the  relative  affinities  of  some  acids,  The  method 
depends  upon  the  fact  that  chemical  processes  which  take 
place  in  homogeneous  liquids  generally  cause  changes  in 
volume.  "  Thus,  the  specific  gravity  of  a  normal  caustic 
soda  solution  was  found  to  be  1.04051,  that  of  an  equiva- 
lent solution  of  sulphuric  acid  1.02970,  that  of  an  equiva- 
lent of  nitric  acid  1.03089.  When  equal  volumes  of  soda 
solution  were  mixed  with  each  of  the  acids,  the  specific 
gravity  of  the  sodium  sulphate  solution  was  1.02959,  and 
that  of  the  nitric  solution  1.02633.  Finally,  when  to 
the  solution  of  sodium  sulphate  (2  vol.)  one  equivalent 
(1  vol.)  of  nitric  acid  was  added  the  specific  gravity  be- 
came 1.02781."*  By  means  of  these  figures  it  is  possible 
to  determine  to  what  extent  the  nitric  acid  acts  upon  the 
sulphate,  and  thus  to  draw  conclusions  regarding  the  dis- 
tribution of  the  base  between  the  acids.  This,  it  will  be 
observed,  is  another  method  of  solving  the  problem  which 
was  attacked  by  Thomsen  by  thermo-chemical  methods. 
The  results  reached  by  the  volume- chemical  method  agree 
in  general  with  those  reached  by  the  thermo-chemical 
method. 

Specific  Coefficient  of  Affinity. — According  to  Guldberg 
and  Waage,  the  coefficient  of  affinity,  k,  in  the  equations 
already  explained,  is  a  coefficient  dependent  upon  the 
chemical  nature  of  the  two  substances  that  enter  into 
action  and  upon  the  temperature.  But  this  coefficient 
must  be  the  product  of  two  specific  coefficients  of  affinity, 
one  belonging  to  one  substance  and  the  other  to  the  second 
substance.  By  investigating  the  action  of  a  number  of 
bases  on  one  acid,  and  of  a  number  of  acids  on  one  base, 
by  different  methods,  Ostwald  found  that  the  relative 
affinity  of  acids  is  independent  of  the  nature  of  the  base, 
and  that  the  relative  affinity  of  the  base  is  independent  of 
the  nature  of  the  acid.  Each  acid  and  each  base  has  a 
specific  coefficient  of  affinity,  and  the  affinity  of  any  acid 
for  a  base  is  the  product  of  the  specific  coefficient  of  affinity 
of  the  acid  and  the  specific  coefficient  of  affinity  of  the  base. 
On  comparing  the  figures  representing  the  specific  coeffi- 
cients of  affinity  of  the  acids  with  the  figures  obtained  by 
Thomsen,  and  called  by  him  the  avidities  of  the  acids,  the 

*  See  Ostwald,  Allgemeine  Chemie. 


304    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

two  are  found  to  bear  approximately  the  same  relation  to 
one  another. 

Methods  for  Determining  Specific  Coefficients  of  Affinity. — 
The  volume-chemical  method  was  supplemented  by  Ost- 
wald  by  several  other  methods  which  enabled  him  to 
determine  the  relative  influences  exerted  by  a  number  of 
acids  under  a  variety  of  circumstances.  Among  other 
methods  may  be  mentioned  the  optical,  the  action  of  acids 
on  insoluble  salts,  contact  action,  the  electrical  method, 
and  the  inverting  action  of  acids  on  sugar.  The  object  in 
view  was  in  all  cases  practically  the  same — to  compare  the 
influence  exerted  by  different  acids  under  the  same  circum- 
stances, and  thus  to  measure  their  specific  coefficients  of 
affinity. 

(1)  In  the  optical    method  the  coefficient  of  refraction 
of  various    solutions   is  determined,  and  the  change  pro- 
duced by  mixing  these   solutions  in  certain  ways,  and  thus 
it  is  possible  to  draw  conclusions  in  regard  to  the  character 
of  reactions  which  take  place  in  solutions. 

(2)  An  illustration  of  ;the  method  involving  the  action 
of  acids  on  insoluble  salts  will  make  the  method  clear.     A 
weighed  quantity  of  calcium  oxalate  is  treated  with  equiva- 
lent quantities  of   different  acids  in  dilute  solutions,  and 
the  quantity  of  the  salt  dissolved  then  determined.     From 
this  it  is  possible  to    calculate  the  specific    coefficients  of 
affinity  of  the  acids. 

(3)  The  simplest   method  of  all  is  the  electrical.    This 
consists  in  determining  the   conducting  power  of  solutions 
of  different  dilutions.      In  this  way  figures  are  obtained 
which  bear  to  one  another   the  same  relations  as  those  ex- 
pressing the  coefficients  of  affinity. 

For  details  in  regard  to  these  and  the  other  methods 
used  by  Ostwald  in  his  studies  of  affinity  the  student  is 
referred  to  the  original  papers  of  this  chemist,  and  to  his 
masterly  book  Allgemeine  Chemie.  It  is  sufficient  for  the 
present  purpose  to  call  attention  to  the  general  result  that, 
when  acids  and  bases  are  compared  in  many  different  ways, 
they  are  found  to  differ  markedly  from  one  another,  and 
the  order  in  which  they  are  arranged  by  the  results  of  the 
different  methods  is  always  practically  the  same. 

Special  attention  should  be  directed  to  one  point  in  this 
connection.  According  to  Arrhenius,  the  electrical  con- 


THE  STUDY  OF  CHEMICAL  AFFINITY.         3Q5 

ductivity  of  solutions  depends  upon  the  extent  to  which 
the  substances  in  solution  are  dissociated  into  ions.  Those 
acids  which  in  general  exhibit  the  greatest  activity  appear 
to  be  most  readily  dissociated  in  water  solution.  It,  there- 
fore, follows  that  the  strength  of  an  acid  depends  upon 
the  ease  with  which  it  is  broken  down  into  its  constituent 
ions. 


306    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 


CHAPTER    XIX. 

CONNECTION    BETWEEN   THE   CHEMICAL   CONSTITUTION 
AND    PROPERTIES   OF   COMPOUNDS. 

General. — In  the  chapter  on  constitution  frequent  refer- 
ence was  made  to  the  fact  that  a  certain  constitution  always 
carries  with  it  certain  properties.  Thus  all  alcohols  have 
certain  common  properties,  and  they  have  the  same  general 
constitution;  and  this  is  true  of  every  other  class  of  com- 
pounds. Besides  this  general  connection,  there  are  more 
special  kinds  which  have  not  thus  far  been  treated.  These 
special  kinds  of  connection  may  be  conveniently  discussed 
under  a  few  heads : — 

(1)  Change  of  character  in  certain  parts  of  a  compound 
caused  by  the  introduction  of  some  atom  or  group ; 

(2)  Tendency  on  the  part  of  certain  compounds  to  break 
down  in  certain  ways ; 

(3)  Influence  exerted  by  certain  atoms  or  groups  already 
in  a  compound  on  the  constitution  of  the  products  formed 
by  further  acts  of  substitution  ; 

(4)  Relative  ease  with  which  isomeric  compounds  enter 
into  action. 

1.   Change  of  Character  in  certain  Parts  of  a  Compound 
caused  by  the  Introduction  of  some  Atom  or  Group. 

Bases,  Alcohols,  Acids. — These  three  classes  of  compounds 
illustrate  very  well  the  marked  changes  in  properties  which 
one  part  of  a  compound  can  undergo  in  consequence  of 
changes  in  some  other  part.  In  the  primary  alcohol, 
R— C — 0 — H,  hydrogen  is  in  combination  with  the  carbon 


atom  with  which  the  hydroxyl  is  combined,  and  the  sub- 
stance has  basic  properties.  When,  however,  oxygen  is 
substituted  for  the  two  hydrogen  atoms  of  the  group  — C— 

II 
H2 


PEOPEETIES  OF  COMPOUNDS.  3Q7 

the  hydroxyl  hydrogen  acquires  entirely  different  proper- 
ties. The  compound  R — C — O — H  is  an  acid.  Here, 

II 

O 

apparently,  the  hydroxyl  remains  in  the  compound  as  it 
was,  but  the  carbon  with  which  it  is  in  combination,  instead 
of  being  linked  to  hydrogen,  as  in  the  alcohol,  is  linked  to 
oxygen.  This  power  of  oxygen  to  give  acid  properties  to 
hydroxyl  is  also  seen  in  metaluminic  acid,  OAl(OH),  ferric 
acid,  O2Fe(OH)2,  and  chromic  acid,  O2Cr(OH)2;  and,  in- 
deed, in  nearly  all  the  common  mineral  acids,  as  phosphoric 
acid,  OP(OH)3,  nitric  acid,  O2N(OH),  sulphuric  acid, 
O2S(OH)2,  chlorous  acid,  OCl(OH),  chloric  acid,O2Cl(OH), 
perchloric  acid,  O3C1(OH),  etc.  In  the  case  of  any  ele- 
ment it  is  true,  with  a  very  few  exceptions,  that  that  acid 
derivative  which  contains  the  largest  proportion  of  oxygen 
has  the  most  marked  acid  character. 

Influence  of  Acid  Groups  like  N0r — An  influence  similar 
to  that  referred  to  in  the  last  paragraph  is  exerted  in  some 
cases  by  the  acid  residue  NO2.  This  is  seen  in  the  acid 
character  of  the  nitro-phenols  and  similar  compounds. 
Phenol  itself  has  very  weak  acid  properties.  It  forms  salts 
when  treated  with  the  caustic  alkalies,  but  it  has  not  the 
power  to  decompose  carbonates.  When,  however,  the  nitro- 
group  is  substituted  for  part  of  the  hydrogen  of  the  benzene 
ring,  the  products  formed  have  acid  properties  which  are 
much  more  marked  than  those  of  phenol.  Tri-nitro  phenol, 
C6H2(NO2\OH,  is  a  strong  acid.  A  similar  influence, 
though  less  marked,  is  exerted  by  chlorine  and  bromine. 

The  influence  of  the  nitro-group  on  the  hydroxyl  group 
of  the  alcohols  proper  is  not  known,  as  nitro-derivatives 
of  these  alcohols  are  not  known.  A  remarkable  influence 
is  exerted  by  this  group  on  hydrogen  in  direct  combination 
with  carbon.  This  is  seen  in  some  of  the  nitro-paraffins. 
H 


The  compound  CH3 — C — NO2  has  acid  properties.     The 

H 

acid   character   is   strengthened    by  the    introduction   of 
bromine    in    the    position    indicated     in    the     formula 


308    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

Br 
CH, — C — NO2.      On    the     other    hand,    the     compound 

H 
OH, 

NO2 — C — CH3  is  not  an  acid,  and  this  is  probably  to  be 

OHS 

explained  by  the  fact  that  there  is  no  hydrogen  in  direct 
combination  with  the  carbon  with  which  the  nitro- group 
is  combined.  Again,  isonitro-propane,  CH3 — CH(NO2) — 
CH3,  has  acid  properties,  while  bromisonitro-propane, 
which  has  been  shown  to  have  the  constitution  CH3 — 
CBr(NO2)— CH3,  has  not. 

Change  in  the  Chemical  Character  of  Ammonia. — Hydro- 
carbon residues  can  be  introduced  into  ammonia  in  place 
of  one  or  more  of  the  hydrogen  atoms  without  causing  any 
serious  change  in  the  character  of  the  compound.  To  be 
sure,  it  makes  a  difference  what  groups  are  introduced. 
Thus  methylamine,  CH3.NH2,  ethylamine,  C2H5.NH2,  and 
other  amines  containing  paraffin  residues  are  strongly 
basic,  while  aniline,  C6H5.NH2,  and  similar  amines  con- 
taining residues  of  the  benzene  hydrocarbons  are  much 
weaker  bases.  If,  instead  of  hydrocarbon  residues,  acid 
residues  are  introduced,  the  change  in  the  character  of 
the  ammonia  is  much  more  marked.  Thus  acetamide, 
C2H3O.NH2,  is  practically  neutral.  If,  further,  an  acid 
residue  is  introduced  in  place  of  a  second  hydrogen  of  the 
ammonia  the  product  is  distinctly  acid,  in  some  cases  even 
strongly  so.  Thus  diacetamide,  (C2H3O)2NH,  has  acid 
properties.  This  is  also  true  of  the  imides  of  dibasic 

CH2.CO 
acids,     like     succinimide,      I  /NH,     phthalimide, 

CH..CCK 

CO  CO 

C6H4/       XNH,   benzoic    sulphinide,  C6H,/        XNH, 

and  many  similar  compounds.  If  in  aniline  acid  atoms  or 
groups  are  substituted  for  a  part  of  the  hydrogen  of  the 
benzene,  the  basicity  of  the  compound  is  changed,  and  the 


PROPERTIES  OF  COMPOUNDS.  309 

extent  of  the  change  is  more  or  less  dependent  upon  the 
position  occupied  by  the  substituting  atom  or  group.  Thus, 
of  the  three  chlor-anilines,  the  para-modification  is  a 
stronger  base  than  the  ortho-  or  meta  modification. 

Influence  of  the  Nitro-  group  on  Chlorine.  —  Chlorbenzene 
is  a  comparatively  stable  compound.  When  treated  with 
alcoholic  ammonia  it  undergoes  no  change.  If,  however, 
nitro-groups  are  introduced  into  it,  the  chlorine  is  easily 
replaced  by  the  amido-group  by  simply  treating  with  alco- 

Cl 
holic  ammonia.      Thus  the  compound  C6H3x  NO2(o)    is 


( 
converted  into  the  compound  C,H,  -<  NO2(o)  . 

UO.GO 

Substitution  in  Hydrocarbons.  —  In  general  the  hydrogen 
of  the  paraffins  is  removed  with  difficulty  and  that  of  the 
benzene  hydrocarbons  with  ease.  Here  evidently  the  ring 
form  of  combination  has  a  decided  influence  on  the  sta- 
bility of  the  compounds.  The  substitution-  products  ob- 
tained from  the  benzene  hydrocarbons  are  more  stable 
than  those  obtained  from  the  paraffins.  Substitution  takes 
place  more  easily  in  a  benzene  hydrocarbon  which  con- 
tains one  or  more  paraffin  residues  than  in  benzene  itself. 
Thus,  while,  when  nitric  acid  acts  directly  upon  benzene, 
only  two  nitro-groups  are  introduced  except  with  great 
difficulty,  there  is  no  difficulty  in  making  tri-nitroxylene, 
C6H(NO2)3(CH3)2,  and>tri-nitromesitylene,C6(NO2)3(CH3)3. 
So  also  tri-nitrophenol  is  made  with  ease. 

A  most  remarkable  difference  is  noticed  between  the 
conduct  of  chlorine  towards  a  homologue  of  benzene  in  the 
light  and  in  the  dark.  In  the  case  of  toluene,  C6H5.CH3, 
for  example,  it  has  been  shown  that,  when  chlorine  acts 
upon  this  hydrocarbon  in  the  direct  sunlight,  substitution 
takes  place  in  the  paraffin  residue  or  methyl,  CH3,  while, 
if  the  action  takes  place  in  the  dark,  the  substitution  is 
confined  to  the  benzene  residue.  So,  too,  there  is  a  similar 
difference  observed  between  the  results  obtained  when  the 
action  takes  place  at  the  boiling  temperature  and  at  the 
ordinary  temperature.  In  the  former  case  the  substitution 


310    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

is  in  the  paraffin  residue ;  in  the  latter  it  is  in  the  benzene 
residue. 

Oxidation-phenomena. — In  general  the  paraffins  are 
more  easily  broken  down  by  oxidizing  agents  than  the  ben- 
zene hydrocarbons.  If  the  residues  of  both  these  classes 
of  hydrocarbons  are  combined  in  one  compound,  the  paraffin 
portion  is  broken  down  by  oxidizing  agents,  while  the  ben- 
zene portion  is  not  affected.  If  a  paraffin  is  oxidized  alone, 
the  products  are  carbon  dioxide  and  water.  The  simplest 
case  is  that  of  marsh-gas,  CH4.  The  changes  involved  in 
the  conversion  of  this  substance  into  carbon  dioxide  and 
water  have  already  been  discussed  (see  ante,  p.  155). 

If  a  stable  benzene  residue  is  combined  with  methyl  or 
any  other  paraffin  residue,  the  latter  breaks  down,  tending 
to  form  carbonic  acid ;  but  as  the  benzene  residue  is  not 
broken  down  the  product  of  the  oxidation  is  carbonic  acid, 
in  which  the  group  C6H5,  or  phenol,  is  present  in  place  of 
one  of  the  hydroxyls : — 

(C6H5 

is  converted  into       C  •<  O 
(OH 

If  a  second  atom  or  group  is  introduced  into  benzene 

f   /">tTT 

forming  a  compound  of  the  general  formula  C6H3  j  -^  3    , 

the  resistance  of  the  paraffin  residue  to  the  influence  of 
oxidizing  agents  depends  upon  the  constitution  of  the 
product.  It  has  been  shown  that,  when  the  group  X  is 
in  the  ortho-position  relatively  to  the  methyl,  an  acid  oxi- 
dizing agent  like  chromic  acid  does  not  change  the  methyl, 
this  residue  becoming  under  these  conditions  as  stable  as 
the  benzene  residue.  On  the  other  hand,  potassium  per- 
manganate oxidizes  such  a  group  without  difficulty.  If 
two  paraffin  residues  are  in  a  benzene  compound  and  a  sub- 
stituting atom  or  group  is  in  the  ortho-position  to  one  and 
not  to  the  other,  the  latter  is  oxidized  by  chromic  acid  and 
the  former  is  not.  Potassium  permanganate  oxidizes  both. 
If  the  compound  is  treated  with  fusing  caustic  potash,  the 
ortho-methyl  is,  however,  first  oxidized. 


PROPERTIES  OF  COMPOUNDS.  .      31 1 


2.   Tendency  on  the  Part  of  Certain  Compounds  to  break 
down  in  Certain  Ways. 

Anhydrides. — The  phenomena  included  under  this  head 
are  illustrated  by  the  formation  of  anhydrides  of  acids  of 
certain  constitution,  The  simplest  cases  are  those  of  car- 
bonic acid  and  sulphurous  acid;  and  the  formation  of  am- 
monia from  ammonium  hydroxide  is  a  reaction  of  the 
same  kind.  In  the  case  of  the  formation  of  carbonic  an- 
hydride, it  is  said  that  one  carbon  atom  cannot  hold  in 
combination  more  than  one  hydroxyl  group.  While  most 
facts  are  in  accordance  with  this  statement,  some  are  not, 
Thus  it  appears  that  in  mesoxalic  acid  there  is  a  carbon 
atom  in  combination  with  two  hydroxyls,  as  represented 

(  PO  IT 
in  the  formula  C(OH)2  j  £Q2jj.     So,  too,  chloral  hydrate 

probably  has  "the  constitution  CC13 — CH(OH)2.  From 
these  facts  it  would  appear  that  carbon,  which  is  in  com- 
bination with  certain  acid  groups,  can  hold  two  hydroxyls 
in  combination.  The  breaking  down  of  sulphurous  acid 

( "FT 

SO2   •!  QJJ  into  sulphur  dioxide  and  water  is  similar  to 

the  breaking  down  of  carbonic  acid.  But  that  sulphur 
can  hold  two  and  perhaps  a  larger  number  of  hydroxyl 
groups  in  combination  is  shown  in  sulphuric  acid,  H2SO4, 
and  in  the  acid  H2SO4  +  2H2O  =  S(OH)6.  Indeed,  the 
two  hydroxyls  in  the  acid  H2SO4  are,  as  is  well  known, 
held  very  firmly.  So,  too,  nitrous  acid,  ON. OH,  loses 
water,  spontaneously  forming  the  anhydride  N2O3,  but 
nitric  acid,  O2N.OH,  does  not  break  down  by  loss  of  water, 
the  addition  of  oxygen  in  this  case  as  in  the  case  of  sul- 
phuric acid  increasing  the  stability  of  the  product.  Facts 
of  the  same  order  are  noticed  in  the  seventh  group  of  ele- 
ments, and  particularly  clearly  among  the  acids  of  iodine. 
The  loss  of  water  and  formation  of  acid  anhydrides  by 
the  interaction  of  two  hydroxyl  groups  not  in  combination 
with  the  same  atom  are  illustrated  in  the  case  of  pyro- 
sulphuric  acid  and  pyrochromic  acid.  The  change  in  the 
latter  case  takes  place  spontaneously,  and,  indeed,  unless 
basic  compounds  are  present,  it  is  carried  to  the  formation 
of  chromic  anhydride.  Among  the  acids  of  silicon  such 
changes  are  of  special  interest,  as  they  give  rise  to  the 


312    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

many  forms  of  polysilicic  acids.  Among  carbon  com- 
pounds a  connection  has  been  established  between  the  con- 
stitution of  certain  acids  and  the  ease  with  which  they 
give  up  water.  The  generalization  which  has  been  reached 
is  this:  When  two  carboxyl  groups  are  in  combination 
with  carbon  atoms  which  are  in  combination  with  each 
other,  water  is  given  off  easily  and  an  anhydride  formed. 

CH2  COOH 

This  is  illustrated  by  the  case  of  succinic  acid,  |  , 

CH2.COOH 
which,  when  heated,  loses  water  and  is  converted  into  suc- 

CH2.CO 
cinic  anhydride,   |  /O  .      In   the  aromatic  series 

CH2.CCK 

this  phenomenon  is  of  special  interest.  It  is  noticed 
that  the  ortho-dicarbonic  acids,  like  phthalic  acid, 

COOH 
C6H4(^  ,   in   which,   according    to    the   commonly 

XCOOH 

accepted  hypothesis  concerning  the  structure  of  benzene, 
the  two  carboxyl  groups  are  in  combination  with  carbon 
atoms  which  are  linked  together,  it  is  noticed  that  such 
acids  lose  water  easily,  while  the  isomeric  acids  do  not 
form  anhydrides.  Thus,  ortho-phthalic  acid  yields  the 

CO 
anhydride  C6H4^         /O  ,    while   isophthalic    and    tere- 

XCOX 
phthalic  acids  do  not  yield  anhydrides. 

Lactones. — The  formation  of  lactones  (see  ante,  p.  219) 
by  loss  of  water  from  the  hydroxyl-acids  is  an  illustration 
of  the  same  kind  of  action  as  that  of  the  formation  of 
acid  anhydrides.  It  has  been  shown  that  the  -y-  and 
d-hydroxyl-acids  are  extremely  unstable,  breaking  down  into 
lactones  and  water  when  set  free  from  their  salts.  Here, 
it  will  be  observed,  the  position  of  the  hydroxyl  with 
reference  to  the  carboxyl  is  of  marked  influence  on  the 
stability  of  the  compounds. 

Lactams  and  Lactims. — Among  aromatic  compounds 
containing  the  amido-group  those  in  which  this  group  is 
in  the  ortho  position  relatively  to  a  group  containing  car- 
boxyl give  up  water  in  two  ways  as  indicated  in  the  two 
equations : — 


PROPERTIES  OF  COMPO  UNDS.  31  3 

CO.COOH  C  O.  CO 


CO.CO.OH  CO 


The  product  of  the  first  reaction  is  called  a  lactam  and 
that  of  the  second  reaction  is  called  a  lactim.  A  condi- 
tion of  the  formation  of  compounds  like  these  is  that  the 
amido-group  must  be  in  the  ortho  position  relatively  to 
the  group  containing  the  carboxyl. 

Other  Anhydro  Compounds.  —  There  are  several  other 
classes  of  compounds  formed  by  loss  of  water  in  much  the 
same  way  that  anhydrides,  lactones,  lactams,  and  lactims 
are  formed.  Among  these  are  the  sulphinides,  which  are 
formed  by  loss  of  water  from  ortho-sulphamine  acids  of 
the  aromatic  series.  The  simplest  case  is  that  of  benzoic 

CO 

sulphinide,  C6H4(          )NH,  which  is  formed  from  ortho- 
XSO/ 

COOH 
sulphamine-benzoic   acid,  C6H4(^  .     The  anhydro 

XSO,NH2 

bases  are  formed  whenever  an  acid  residue  is  intro- 
duced into  an  ortho-diamido  compound.  Thus,  when 
ortho-diamido-benzene  is  treated  with  glacial  acetic 
acid,  the  product  is  a  compound  of  the  constitution 


C6H,^        ^C.CH3.     The  formation  is  due  to  a  loss  of 

NH.CO.CH3 
water  from  the  compound  C6H4(^  ,  which  in 


all  probability  is  the  first  product  of  the  reaction. 

Elimination  of  Carbon  Dioxide.  —  A  change  similar  to 
that  referred  to  in  the  last  paragraph  is  the  loss  of  carbon 
dioxide.  This  change  is  not  nearly  as  common  as  the  loss 
of  water.  From  the  facts  known  it  appears  that  an  ac- 
cumulation of  carboxyl  groups  in  combination  with  one 
carbon  atom  gives  rise  to  an  unstable  condition.  One 


314    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

illustration  will  suffice.     Iso-succinic  acid,  which  has  the 

CH(COOH)2 
constitution     |  ,   easily  loses   carbon   dioxide 

CH3 

when  heated,  and  is  thus  converted   into  propionic  acid, 
CH2.COOH 


CH3 


Conclusions  Warranted  by  the  Facts  just  Presented.— 
The  fact  that  ortho-  compounds  in  general  give  up  the  ele- 
ments of  water  more  easily  than  the  compounds  of  the 
meta-  and  para-series  has  led  some  to  conclude  that  in  the 
ortho-compounds  the  substituting  groups  are  nearer  to- 
gether than  in  the  meta-  and  para-compounds.  This  ar- 
gument is  evidently  not  valid.  Proximity  of  two  groups 
containing  the  elements  of  water  is  not  the  main  condition 
for  the  reaction.  This  is  shown  by  the  fact  that  ortho- 

yCOOH 
amido-benzoic    acid,    C6H4^  ,    does    not    easily 

.CO.COOH 
lose    water,    while    the    compounds    C6H4^  , 

XNH2 
/CH2.COOH  ,  /CH2.CH2.COOH 

C6H4(  and   C6H  /  ,    lose 


water  spontaneously.  According  to  our  formulas,  the  car- 
boxyl  and  the  amido  group  arein  closer  proximity  in 
ortho-amido-benzoic  acid  than  in  the  other  three  acids,  the 
formulas  of  which  are  given.  So,  too,  in  the  formation  of 
lactones  proximity  is  not  favorable  to  the  reaction.  It 
does  not  take  place  in  «-  and  /5-hydroxyl  acids  like 
CH3—  CH—  COOH  and  CH2—  CH2.-COOH,  but  does 

I  I 

OH  OH 

a-acid.  8-acid. 

takes  place  in  f-  and  <?-acids  like 
CH2.CH2.CH2.COOH         CH2.CH2.CH2.CH2.COOH. 

OH  OH 

y-acid.  d-acid, 


PROPERTIES  OF  COMPOUNDS.  315 

It  appears  rather  that,  in  order  that  the  reaction  may 
take  place,  it  is  necessary  that  a  number  of  atoms  should 
intervene  between  the  two  hydroxyls.  That  this  number 
is  not  constant  is  clear  from  the  above  cases.  The  expla- 
nation of  facts  of  the  kind  mentioned  appears  to  be  found 
in  the  study  of  the  space-relations. 

Breaking- down  of  Unsaturated  Carbon  Compounds. — 
The  principal  fact  to  be  noted  under  this  head  is  that  when 
an  unsaturated  compound  breaks  down  the  separation  of 
carbon  atoms  generally  takes  place  first  where  the  double 
or  triple  linkages  are  assumed  to  exist.  Thus,  when  treated 
with  fusing  caustic  potash  crotonic  acid,  CH3 — CH— CH 
— COOH,  yields  onlv  acetic  acid ;  methacrylic  acid, 

CH, 

CH2=C^  ,  however,  yields  propionic  acid:  tiglic 

XCOOH 

CH3 
acid,  CH3 — CH— C\  ,  yields  propionic  and  acetic 

XCOOH 

acids;  hydrosorbic  acid,  CH3— CH=CH— CH2—  CH2— 
COOH,  yields  normal  butyric  acid  and  acetic  acid.  These 
reactions  are  clear,  if  it  is  assumed  that  the  double  link- 
age is  the  first  to  give  way,  and  that  the  parts  thus  formed 
are  converted  into  saturated  monobasic  acids  of  the  fatty 
acid  series. 


3.  Influence  exerted  by  certain  Atoms  or  Groups  in  a  Com- 
pound on  the  Constitution  of  the  Products  formed  by  further 
Acts  of  Substitution. 

Substitution  in  Symmetrical  Compounds. — When  substi- 
tution of  one  hydrogen  in  a  symmetrical  compound  takes 
place,  but  one  product  can  be  formed.  In  the  foregoing 
it  has  been  pointed  out  that  some  of  our  fundamental 
views  regarding  the  structure  of  the  compounds  of  carbon 
are  based  upon  this  proposition.  The  fact  that  marsh-gas, 
ethane,  and  benzene  each  gives  but  one  variety  of  mono- 
substitution-products  is  the  strongest  argument  in  favor 
of  the  view  that  these  compounds  are  symmetrical,  or  that 
the  hydrogen  atoms  in  each  of  them  bear  the  same  relation 
to  the  molecule.  The  moment  substitution  has  taken  place 


316    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

in  a  symmetrical  compound  and  a  mono-substitution- pro- 
duct is  formed,  the  symmetry  is  destroyed,  and,  in  all  cases 
except  that  of  marsh  gas,  the  second  substituting  atom  or 
group  may  enter  in  more  than  one  place.  Thus,  when 
chlorine  has  been  substituted  for  hydrogen  in  ethane,  the 
unsymmetrical  compound  CH3 — CH2C1  is  formed.  A 
second  chlorine  may  enter  either  in  the  position  indicated 
in  the  formula  CH3 — CHC12  or  in  that  indicated  in  the 
formula  CH2C1 — CH2C1.  In  mono-chlorbenzene  chlorine 
may  enter  in  three  positions  indicated  by  the  formulas : — 


CH 


Some  progress  has  been  made  in  the  investigation  of 
these  phenomena,  and  a  few  generalizations  have  been 
reached. 

Influence  of  Acid  or  Negative  Groups  on  Groups  of  the 
same  Kind. — The  most  common  case  is  that  in  which  an 
acid  atom  or  group  enters  a  compound  in  which  there 
is  already  an  atom  or  group  of  the  same  kind.  So  far  as 
this  subject  has  been  investigated  in  the  paraffin  series  it 
appears  .that  a  second  acid  atom  entering  a  compound 
tends  to  combine  with  the  same  carbon  as  that  with  which 
the  first  is  combined.  The  same  rule  appears  also  to  apply 
to  the  entrance  of  a  third  acid  atom.  Thus,  by  the  action 
of  chlorine,  chlor-ethane,  CH3 — CH2C1,  is  converted  suc- 
cessively into  CH3— CHC12,  CH3— CC13,  CH2C1-CC13,  etc. 
The  course  of  these  reactions  is,  however,  dependent  upon 
the  conditions.  Under  some  conditions  chlor-ethane  is 
converted  into  ethylene  chloride,  CH2C1 — CH2C1,  by  the 
action  of  chlorine.  The  following  are  some  of  the  general 
laws  that  have  been  found  to  govern  substitution  in  the 
aromatic  series: — 

(a)  When  chlorine  or  bromine  is  present  in  benzene  a 
second  atom  of  either  of  these  elements  takes  mainly  the 


PROPERTIES  OF  COMPOUNDS.  317 

para-position,  though  there  is  always  substitution  to  a 
slight  extent  in  the  ortho-position. 

(b)  A  second  nitro-group  takes  mainly  the  meta-posi- 
tion  with  reference  to  the  first,  though  to  some  extent  it 
takes  the  ortho-  and  para-positions. 

(c)  A  second  sulphonic  acid  group  (SO3H)  takes  mainly 
the  para-position,  but  to  some  extent  the  ortho-position. 

(d)  Toward  hydrocarbon  residues  like  CH3,  C2H5,  etc., 
acid  groups  tend  to  take  the  para-position.     Ortho-com- 
pounds are  formed  in  smaller  quantity. 

(e)  Hydroxyl  exerts  in  general  the  same  kind  of  influ- 
ence as  methyl  and  other  hydrocarbon  residues. 

(f)  Carboxyl  tends  to  make  acid  groups  take  the  meta- 
position  mainly,  though  in  some  cases  the  ortho-  and  para- 
positions  may  be  occupied  to  some  extent.     When  chlorine 
acts  upon  benzoic  acid,  meta  chlor-benzoic  acid  is  appar- 
ently the  only  mono-substitution-product  formed;    when 
sulphuric  acid  acts  upon  benzoic  acid  meta-sulpho-benzoic 
acid  is  the  principal  product,  but  there  is  formed  at  the 
same  time  a  small  quantity  of  the  para-acid ;  and,  finally, 
when  nitric  acid  is  the  substituting  agent  the  meta-acid  is 
the  chief  product,  and  the  ortho-  and  para-acids  are  formed 
in  smaller  quantity. 

Influence  of  Basic  or  Positive  Groups. — The  ammonia 
residue  NH2  is  the  most  common  example  of  such  groups. 
The  influence  of  this  group  is  such  as  to  make  an  acid 
group  take  mainly  the  para-  and  meta-positions,  and,  to  a 
subordinate  extent,  the  ortho-position. 

Other  regularities  besides  those  above  mentioned  have 
been  observed,  and  the  general  subject  of  such  regularities 
is  under  investigation.  Still,  we  are  only  at  the  beginning 
of  our  knowledge  of  these  phenomena,  and  much  is  to  be 
hoped  for  from  a  more  careful  study  of  substitution- phe- 
nomena from  this  point  of  view.  The  above  statements 
are  not  all  absolutely  true,  for  in  some  cases  the  nature  of 
the  substitution-products  is  found  to  differ  according  to  the 
conditions  under  which  they  are  formed.  Thus,  though  at 
ordinary  temperatures  the  chief  product  of  the  action  of 
sulphuric  acid  on  phenol  is  the  ortho-sulphonic  acid,  the 
para-acid  being  formed  in  small  quantity,  at  a  higher  tem- 
perature only  the  para  acid  is  formed. 


318    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

Regularities  in  the  Addition  of  Negative  or  Add  Atoms  to 
Unsaturated  Compounds. — A  kind  of  action  similar  to  that 
treated  of  in  the  last  paragraph,  though  at  the  same  time 
different  from  it,  is  that  of  the  formation  of  substitution- 
products  of  saturated  hydrocarbons  by  the  addition  of 
negative  atoms.  Thus,  as  has  been  shown  (see  ante,  p. 
229),  when  ethylene  is  treated  with  bromine,  ethylene  bro- 
mide, C2H4Br2,  is  formed;  when  it  is  treated  with  hydro- 
bromic  acid,  bromethane,  C2H5Br,  is  formed.  Whenever 
action  of  this  kind  takes  place,  one  atom  or  univalent  group 
is  added  to  each  of  the  carbon  atoms  which  are  doubly 
linked  together.  Taking  now  such  cases  as  propvlene, 
CH3— CH=CH2,  and  butylene,  CH3— CH2-CH=CH2,  it 
will  be  seen  that  the  addition  of  a  halogen  acid  may  take 
place  in  two  ways  represented  by  these  equations: — 

(1)  CH3— CH=CH2  +  HBr  =  CH3— CH2-  CH2Br  ; 

(2)  CH3— CH=CH2 -{- HBr  =  CH3— CHBr—  CH3; 

(3)  CH3— CH2— CH=CH2-f-HBr  = 

CH3— CH2-CH2— CH2Br  ; 

(4)  CH3— CH2— CH= CH2  +  HBr  = 

CH3— CH2— CHBr— CH3. 

As  a  matter  of  fact,  the  addition  takes  place  according 
to  the  equations  (2)  and  (4) ;  or,  when  a  halogen  acid  is 
added  to  an  unsaturated  compound,  the  hydrogen  is  added 
to  that  one  of  the  doubly  linked  carbon  atoms  which  already 
is  in  combination  with  the  most  hydrogen.  This  suggests 
the  action  in  the  case  of  chlorine.  It  was  found  that  chlo- 
rine enters  into  combination  with  that  carbon  which  already 
is  in  combination  with  chlorine. 


4.  Relative  Ease  with  which  Isomeric  Compounds  enter  into 
Action. 

Velocity  of  the  formation  of  Ethereal  Salts. — The  action 
of  alcohols  upon  acids  has  already  been  referred  to  as  fur- 
nishing a  method  of  studying  the  velocity  of  chemical 
changes.  Menschutkin  has  investigated  this  subject  with 
reference  to  the  connection  between  constitution  and  ve- 
locity of  chemical  change.  He  treated  equivalent  quan- 
tities of  various  alcohols  of  the  methyl  alcohol  series  with 
acetic  acid  at  155°  and  determined  (1)  the  extent  of  change 
at  the  end  of  the  first  hour ;  and  (2)  the  limit  of  change 


PROPERTIES  OF  COMPOUNDS.  319 

when  equilibrium  was  reached.  The  first  result  expressed 
in  percentages  is  called  velocity,  the  second  the  limit.  As 
illustrations  of  the  results  the  following  are  given : — 

Velocity.        Limit. 

{  Ethyl  alcohol  46.8  69.6 

Primary           Propyl    "  46.5  69.9 

Alcohols    1  Butyl      "  46.9  67.3 

tOctyl      "  46.6  72.3 

{  Dimethyl- carbinol  26.5  60.5 

Ethyl-methyl-carbinol  22.6  59.3 

Hexyl-methyl-carbinol  21.2  62.0 

Isopropyl-methyl-carbinol  19.0  59.3 

Diethyl-carbinol  16.9  58.6 

The  figures  express  (1)  the  percentage  of  the  acid  trans- 
formed in  one  hour ;  (2)  the  percentage  of  the  acid  trans- 
formed when  action  ceases,  or  when  equilibrium  is  estab- 
lished. It  will  be  observed  that  there  is  a  marked  difference 
between  the  figures  obtained  with  the  primary  alcohols 
and  those  obtained  with  the  secondary  alcohols.  The  ter- 
tiary alcohols  were  found  to  have  a  very  small  velocity. 
Similar  differences  were  observed  between  tertiary  acids 

CH3 

like  trimethyl-acetic  acid,  CH3 — C — COOH,  and  primary 

CH3 

and  secondary  acids,  while  between  the  primary  and  sec- 
ondary acids  themselves  very  slight  differences  were  ob- 
served. 

Formation  of  Ethereal  Salts  of  Certain  Aromatic  Acids. — 
Victor  Meyer  has  shown  that  certain  aromatic  acids  resist 
the  action  of  alcohols  in  a  very  remarkable  way.  The 
results  reached  by  him  may  be  summed  up  in  a  few  words 
thus:  An  aromatic  acid  containing  two  atoms  or  groups, 
both  in  the  ortho-position  relatively  to  the  carboxyl,  is 
practically  not  acted  upon  by  an  alcohol  in  presence  of 
hydrochloric  acid,  whereas  an  acid  containing  the  carboxyl 
in  any  other  position  is  readily  converted  into  an  ethereal 
salt  under  these  conditions. 


320    PRINCIPLES  OF  THEORETICAL  CHEMISTRY. 

Decomposition  of  Halogen  Derivatives.  —  Some  rough  ob- 
servations have  been  made  on  the  time  required  to  effect 
complete  decomposition  of  certain  halogen  derivatives  of 
the  paraffins.  The  reaction  made  use  of  was  that  which 
takes  places  when  the  sodium  compound  of  aceto-acetic 


,  C 


ether,  CHNa     ,  is  treated  with  a  halogen  compound  like 


methyl  iodide,  and  represented  thus  :  — 
CH3  CH3 

CO  CO 

I  I 

CHNa  +  ICH3  =  CH.CH3  +  Nal. 


C02C2H5 


The  reaction  was  tried  with  methyl  iodide,  ethyl  iodide, 
ethyl  bromide,  propyl  iodide,  and  isopropyl  iodide.  The 
time  required  to  effect  complete  transformation  of  molec- 
ular weights  of  these  compounds  in  grams  was  :  — 

Methyl  iodide,  4  minutes 

Ethyl  iodide,  39 

Ethyl  bromide,  460       " 

Propyl  iodide,  162       " 

Isopropyl  iodide,  445       " 

Similar  experiments  have  been  made  with  some  of  the 
same  halogen  derivatives  and  nascent  hydrogen  from  dif- 
ferent sources,  with  silver  nitrate  and  with  caustic  alkalies. 
While  the  results  cannot  be  stated  quantitatively  with  any 
degree  of  confidence,  one  general  result  is  of  interest.  It 
appears  that:  In  acid  solution  ethyl  and  normal  propyl 
bromides  are  most  stable,  and  isopropyl  bromide  least  so. 
In  alkaline  solutions,  on  the  contrary,  the  reverse  is  true, 
that  is  to  say,  ethyl  and  normal  propyl  bromides  are  least 
stable,  and  isopropyl  bromide  most  stable. 


INDEX, 


A  CETONES,  152,  215 
jLi.     methods  of  formation,  153 
Acetoximes,  171 
Acetylene.  235 
Acid,  acetic,  211 

acrylic,  234 

butyric,  211,  213 

caproic,  215 

carbonic,  225 

chlorauric,  126 

chloric,  176 

chlorous,  176 

chlorplatinic,  126 

chromic,  197 

cinnamic,  262 

citric,  222 

cyanic,  223 

dithionic,  183 

formic,  311 

fumaric,  289 

glyceric,  221 

gly colic,  217 

hydracrylic,  218 

hydrazoic,  125 

hydrotiglic,  214 

hydroxyacetic,  218 

hydro xypropionic,  217 

a-hydroxypropionic,  218 

hypochlorous,  176 

hypophosphorous,  186 

isobutyric,  213 

isophthalic,  247,  249,  251 

isosuccinic,  220 

iso  valeric,  214 

Isevolactic,  218 

maleic,  289 

malonic,  220 

mesitylenic,  251 

metaphosphoric,  191.   *  /••  * 

methylethy  lace  tic,  214  •  • 


Acid,  nitric,  185 
nitrous,  185 
normal  boric,  191 
silicic,  192 
valeric,  214 

orthoamidobenzoic,  262 
oxalic,  219 
oxybenzoic,  251 
paralactic,  218 
paroxybenzoic,  251 
pentath  ionic,  184 
perchloric,  176 
periodic,  176 
phosphoric,  188 
phosphorous,  187 
phthalic,  247,  251 
propionic,  211 
pyrochromic,  198 
pyromucic,  263 
pyrophosphoric,  190 
pyrosulphuric,  182 
salicylic,  251 
silicic,  192 
succinic,  220 
sulphocarbonic,  227 
sulphocyanic,  223 
sulphuric,  178 
sulphurous,  177 
tartaric,  222 

terephthalic,  247,  249,  251 
tetrathionic,  177,  184 
thiosulphuric,  182 
trimesitic,  251 
trimethyl-acetic,  214 
triphenylcarbinol-carbonic, 

261 
triphenylmethane-carbonic, 

261 

trithionic,  183 
uric,  226 
:  (321) 


322 


INDEX. 


Acid,  uvitic,  251 

valeric,  211,214 

xanthogenic,  226 
Acids,  121,  146 

avidity  of,  298 

classification  of,  126 

dibasic,  127 

double  halogen,  125 

hydrogen,  121 

hydroxyl,  121 

metal,  197 

methods  of  formation,  147 

monobasic,  127 

monohydroxy  -  monobasic, 
217 

nitrogen,  125 

normal,  193 

polysilicic,  192 

sulphur,  124 

tetrabasic,  127 

tribasic,  127 

unsaturated,  235 
Acrolem,  235 
Affinity,  chemical,  17,  294 

coefficients  of,  300 

distinction  between,  and  va- 
lency, 91 

measurements  of,  by  obser- 
vations on  heat- phenom- 
ena, 296 

resultant,  297 

rough  measurements  of,  294 

specific  coefficient  of,  303 
Alcohol,  active  amyl,  206,  207 

allyl,  233 

ethylene,  215 

isoamyl,  206 

isobutyl,  204 

isopropyl,  202 

normal  amyl,  206 

normal  butyl,  204 

propyl,  202 

pseudopropyl,  202 

secondary  butyl,  204 

tertiary  butyl,  205 
Alcohols,  140 

diacid,  216 

primary,  141 

secondary,  142 

tertiary,  144 

triacid,    222 

unsaturated,  234 


Aldehydes,  149,  215 
Aldoximes,  171 
Alloy,  31,  32 
Allyl-mustard  oil,  224 
Amido-compounds,  172 
Amines,  282 
Ammonium,  195 

chloride,  specific  gravity  of 

vapor,  56 
salts,  195 

Ampere's  hypothesis,  36 
Amylbenzene,  245 
Anhydrides,  130,  159,  310 
Anhydro-bases,  312 
Anhydro-compounds,  312 
Anthracene,  270 
Anthraquinone,  271 
Arrhenius,  63,  64,  304 
Atomic  hypothesis,  14     — -~ 
theory,  21 
weights,    determination    of, 

22 

by  analysis,  22 
by  chemical  decom- 
positions, 29 
by  substitution,  28 
determined   by   isomor- 
phism, 75 
Atoms,  21,  38 

linkage  of,  117 

number   of,  in  molecules  of 

elements,  41 
Avidity  of  acids,  295 
Avogadro's  hypothesis,  37,  38 

law,  39 

Azobenzene,  256 
Azo-compounds,  254 
Azoimide,  125 
Azoxy-compounds,  258 


BAEYEK  on  benzene,  244 
on  ring-compounds,  288 
Bamberger  on  naphthalene,  266 
Bases,  127 

complex,  129 
Benzene,  237 

prism-formula  of,  243 

substitution -products  of,  245 
Berthelot,  278 

and  St.  Gilles,  301 
Berthollet,  19,  299 


INDEX. 


323 


Berzelius,       determination       of 
atomic  weights  by,  26 

rules  of,  27 

Biltz      See  Victor  Meyer. 
Bonds  of  carbon  atoms,  282 
Boyle's  law.  38 
Briihl,  277,  278,  285 
Bunsen  on  mass-action,  300 
Butane,  derivatives  of,  202 

normal,  203 
Butylbenzene,  245 
Butylcarbinol,  206 

secondary,  206 
Butyro-lactone,  219 


PAEBAMIDE,  226 

w     Carbamines,  165 
Carbon,  asymmetrical,  207,  219, 
291 

disulphide,  226 

sulphoxide,  226 
Carbonyl  chloride  226 
Chemical  affinity,  17,  294 
Chromium  salts,  197 
Chrysene,  272 
Glaus  on  constitution  of  benzene. 

244 

Clausius,  63 

Coefficients  of  affinity,  301 
Combining-numbers,  21 
Compound,  chemical,  31 
Compounds,  31 

atomic,  94 

molecular,  94 

saturated,  99,  228 

unsaturated,  99,  228 
Constitution,  definition  of,  111 
Copper  salts,  195 
Crafts,  investigations  on  the  spe- 
cific gravity  of  the  vapor 
of  iodine,  47 

See  Friedel. 
Cyanamide,  223 
Cyanides,  165 
Cyanogen,  223 

compounds,  223 

D  ALTON,  investigations  of,  19 
De  Vries  on  osmotic   pres- 
sure, 63 


Diazo-compounds,  254 
Dichlornaphthoquinone,  265 
Diethyl-carbinol,  206,  207 
Di-isopropyl ,  208 
Dimethyl-butyl-methane,  209 

-diethyl-methane,  209 

-ethyl-carbinol,  207 
-methane  205 
Diphenyl-methane,  259 

-phthalide,  261 
Dipropargyl,  236 
Dissociation  in  solutions,  63,  64 
Dulong  and  Petit,  investigations 
of,  66 

ELEMENTS,  30 
Equivalents,  24 
Ethane,  derivatives  of,  200 
Ethereal  salts,  157 
Ethers,  159 
Ethylbenzene,  245 
Ethylene  232 

chloride,  200 
Ethyl-mustard  oil,  224 

T7ABADAY  on  magnetic  rotary 

polarization,  283 
Fatty  compounds,  1 99 
Favre  and  Silbermann,  43,  278 
Fischer,  E. ,  on  sugars,  288 
Formula,  constitutional,  116 

empirical,  112 

molecular,  of  gaseous  com- 
pounds, 55 

reaction,  115 

structural,  116 

synthesis,  115 

Freezing-point  of  solutions,  61 
Friedel  and  Crafts,  reaction  of, 

259 

Furfural,  263 
Furfuran,  263 

GAY   LUSSAC,  investigations 
of,  34 

Glycerin.     See  Glycerol. 
Glycerol,  221 
Glycol,  216 

Gravity,  specific,  of  compounds 
in  form  of  vapor,  48-52 


324 


INDEX. 


Gravity,  specific,  abnormal,  55 
Guldberg  and  Waage,  297,  300 

HANTZSCH      and      Werner, 
theory  of,  291 
Heat,  molecular,  72 
of  formation,  279 
of  neutralization,  281,  298 
specific,  65 

of  boron,  73 
of  carbon,  73 
of  silicon,  73 
of  vapor  of  mercury,  45 
Heptane,  derivatives  of,  209 

normal,  209 
Hess,  278 
Hexane,  derivatives  of,  208 

normal,  208 
Homologous  series,  138 
Homology,  138 

Horstmann  on  mass-action,  300 
Humpidge,  74 
Hydrazine,  173 
Hydrazines,  173 
Hydrazo-compounds,  258 
Hydrocarbons,    constitution    of, 

136 

normal,  139 
Hydroquinol,  251 
Hydroxylamine,  186 
Hypothesis,  14 

of  Le  Bel  and  Van't  Hoff, 
218,  286 

TMIDO-COMPOUNDS,  172 

1     Indigo-blue,  262 

Iodine,   changes    in,    caused    by 

heat,  47 
Iron  salts,  196 
Isatine,  262 
Isobutane,  203 
Isobutyl-carbinol,  206 
Isocyanides,  165 
Isoheptane,  209 
Isomerism,  201 
Isomorphism,   determination    of 

atomic  weights  by  means  of,  75 
Isonitriles,  165 
Isonotroso-compounds,  171 
Isopropyl-carbinol,  204 
Isoxylene,  251 


KETONES,  216 
Kopp  on  isomorphism,  76 
Kundt   on  specific  heat  of  mer- 
cury vapor,  45 


T  ACTAMS,  312 

Ll    Lactims,  312 

Lactones,  219,  311 

Landolt,  276 

Le  Bel,  hypothesis  of,  218,  286 

Law: 

of   combination  by  volume, 

34 
of  definite  proportions,  14, 

19 
of  indestructibility  of  matter, 

14 
of  multiple  proportions,  14, 

19 

periodic,  78 
Linkage  of  atoms,  117 
double,  102,  231 
single,  102 
triple,  102,  231 


MAGNETIC    rotary    polariza- 
tion, 282 

Mariotte's  law,  40 
Mass  action,  299 
Matter,  constitution  of,  21 
Mechanical  mixture,  31,  32 
Meier    and    Victor     Meyer    on 

specific    gravity   of    vapor   of 

iodine,  47 
Meier,  F.,  and  Crafts  on  specific 

gravity  of  vapor  of  iodine,  47 
Mendeleeff  on  periodic  law,  79 
Menschutkin,  318 
Mercaptans,  146 
Mercury  salts,  195 

specific  heat  of  vapor  of,  45 
Mesitylene,  248,  252 
Meta-compounds,  246 
Meta-dibrombenzene,  251 
Meta-dinitrobenzene,  251 
Metamerism,  201 
VIetapicoline,  268 
Methane,  constitution  of,  136 

derivatives,  199 
Methylbenzene,  245 


INDEX. 


325 


Methyl-diethyl  methane,  208 
-ethyl-carbinol,  204 

-propyl-methane,  210 
-isopropyl-carbinol,  206 
-mustard  oil,  224 
-propyl  carbinol,  206 
Meyer,  Lothar,  on  periodic  law, 

86 
Meyer,  Victor,  on  specific  gravity 

of  vapor  of  iodine,  47 
on  etherification  of  acids,  319 
and  Biltz,  specific  gravity  of 

vapor  of  sulphur,  46 
Mitscherlich,     law    of    isomor- 
phism, 75 

Mixture,  mechanical,  31,  32 
Molecular  rotary  power,  283 

weights,  determination  of,  39 
Molecules,  37,  38 

of   elements  with   more  or 

less  than  two  atoms,  45 
shape  of,  284 
Mustard  oils,  224 


MAPHTHALENE,  247,  265 

II     Nascent  state,  54 

Neuberg,  57 

Neumann  on  specific  heat  of  com- 
pounds, 67 

Neutralization,  heat  of,  281,  298 

Newlands  on  periodic  law,  78 

Nilson,  74 

Nitriles,  165 

Nitro-compounds,  168 

Nitrogen  peroxide  specific  grav- 
ity of,  58 

Nitroso-compounds,  171 

Nitrous  oxide,  185 

Numbers,  combining-,  21 


OILS,  mustard,  224 
Ortho  compounds,  246 
Orthodibrombenzene,  251 
Orthodinitrobenzene,  251 
Orthopicoline,  268 
Ortho-xylene,  251 
Osmotic  pressure,  61 
Ostwald,  300,  302-304 
Oxidation-phenomena,  309 
Oxides,  133 


Ozone,   proof  that  its  molecule 
contains  three  atoms,  54 

PARA-COMPOUNDS,  246 

1      Paradibrombenzene,  251 

Paradinitrobenzene,  251 

Paraleucaniline,  260 

Para-picoline,  268 

Pararosaniline,  259 

Paraxylene,  251 

Pentane,  derivatives  of,  205 
normal,  205 

Periodates,  194 

Periodic  law,  78 

Perkin  on  magnetic  rotation,  283 

Petit.     See  Dulong. 

Petterson.    See  Nilson. 

Pfeffer  on  osmotic  pressure,  60, 
62,  63 

Phenanthrene,  272 

Phenol-phthalein,  261 

Phenols,  253 

Phenylethylene,  262 

Phenylmethanes,  259 

Phosphorus  pentachloride,    spe- 
cific gravity  of  vapor  of,  57 

Phthaleins,  261 

Physical  methods  for  determining 
constitution,  274 

Physical  structure,  274 

Polarization,  magnetic  rotary,  282 

Polymerism,  201 

Propane,  derivatives  of,  201 

Propylbenzene,  245 

Propyl-carbinol,  204 

Propylene,  233 

Proust,  investigations  of,  19 

Pyrene,  272 

Pyridine,  268,  282 

Pyrocatechol,  251 

Pyrrol,  263 

QUINOLINE,  268,  269 
Quinones,  254 

RAOULT  on  solutions,  60 
Refraction-equivalent,  276 
Refraction,  molecular,  276 
Regnault  on  specific  heat  of  com- 
pounds, 67 


15 


326 


INDEX. 


Resorcinol,  251 

Riecke,  46 

Ring-compounds,  Von  Baeyer's 

views  on,  288 
Rosaniline,  259 
Rose  on  mass-action,  300 

SALTS,  129 
complex,  130 
ethereal,  157 
Selenium,    varying    number    of 

atoms  in  molecule,  46 
Silbermann.    See  Favre. 
Solution,  31,  83 
Solutions,  laws  of,  60-62 
Stereochemistry,  286 
Stereoisomerism  due  to  nitrogen, 

290 

St.  Gilles.     See  Berthelot. 
Structure,  definition  of,  111 
Styrene,  262 
Substituting-groups,  constitution 

of,  164 
Substitution,  161  ^ 

in       determining        atomic 

weights,  28 
-products,  complex,  164 

containing        chlorine, 
bromine,    or    iodine, 
m  162 

Succinamide,  264 
Succinic  anhydride,  264,  315 
Sulphur  dioxide,  176 
trioxide,  176 

varying  number  of  atoms  in 
molecule,  46 

rTETRAMETHYL-ETHANE, 
1         208 

-methane,  205 
Tetraphenylmethane,  259 
Theory,  14 

atomic,  21 

of  Guldberg  and  Waage,  303 

of  types,  113 


Thermal  methods,  278 
Thiophene,  264,  282 
Thomsen,  J.,  278,  280-282,  302 
Toluene,  245 
Triethyl-methane,  209 
Trimethyl-carbinol,  205 

-ethyl-methane,  208 

-methane,  203 
Triphenyl-methane,  259 
Types,  113 

mixed,  113 

theory  of,  113 


TT  R  A  N I U  M-COMPOUNDS , 
U     198 
Urea,  226 


YAN'T  HOFF,  hypothesis  of, 
218,  286 

on  solutions,  60,  62,  63 
Valency,  88-110 

apparent,  99 

determination  of,  91 

maximum,  99 

periodic  variations  in,  106 

relative,  104 

variations  in,  94 
Valylene,  236 

Vapor-pressure  of  solutions,  60 
Velocity  of  chemical  change,  302 

of    formation     of     ethereal 

salts,  314 
Volume,  molecular,  274 

specific,  274 


WAAGE.     See  Guldberg. 
Warburg.     See  Kundt. 
Werner.    See  Hantzsch. 
Wollaston,  24,  25 


VYLENES,  249 

A 


CATALOGUE  OF  PUBLICATIONS  OF 

LEA    BROTHERS   &   COMPANY, 

706,  7O8  &  71O  Sansoin  St.,  Philadelphia. 
Ill  Fifth  Ave.  (Cor.  18th  St.),  New  York. 

The  books  in  the  annexed  list  will  be  sent  by  mail,  post-paid,  to  any  Post-Office  in  the 
United  States,  on  receipt  of  the  printed  prices. 

INDEX. 

ANATOMY.     Gray,  p.  11  ;  Treves,  30  ;  Gerrish,  11;  Brockway,  4. 

DICTIONARIES.     Dunglison,  p.  9  ;  Duane,  8  ;  National,  4. 

PHYSICS.     Draper,  p.  8  ;  Martin  &  Rockwell,  19. 

PHYSIOLOGY.    Foster,  p.  10;  Chapman,  5;   Schofield,  25;  Collins 
&  Rockwell,  6  ;  Hare,  12.  [Remsen,  24. 

CHEMISTRY.      Simon,  p.  25  ;  Attfield,  3  ;  Martin  &  Rockwell,  19; 

PHARMACOLOGY.  Cushny,  p.  6. 

PHARMACY.    Caspari,  p.  5. 

MATERIA   MEDICA.     Calbretb,  p.  7  ;   Maisch,  19  ;  Farquharson,  9  ; 

DISPENSATORY.    National,  p.  20.  [Bruce,  4  ;  Schleif,  24. 

THERAPEUTICS.      Hare,  p.  13  ;  Fothergill,  10  ;  Wbitla,  31  ;  Hayem 
&  Hare,  14  ;  Bruce,  4  ;  Schleif,  24  ;  Cushny,  7  ;  Tirard,  29. 

PRACTICE.      Flint,  p.  10  ;     Loomis  &  Thompson,  18  ;     Malsbary,  19  ; 
Thompson,  29. 

DIAGNOSIS.    Musser,  p.  20;  Hare,  13;  Simon,  25;  Herrick,  14;  Hutchi- 
son &  Rainey,  15  ;  Collins,  6. 

CLIMATOLOGY.    Solly,  p.  26  ;  Hayem  &  Hare,  14. 

NERVOUS  DISEASES.    Dercum,  p.  8  ;    Potts,  23. 

MENTAL  DISEASES.     Clouston,  p.  6  ;  Folsom,  10. 

BACTERIOLOGY.       Abbott,  p.  2 ;    Vaughan  &  Novy,  30  ;    Senn's 
(Surgical),  25.      Park,  22  ;  Coates,  6.  [Vale,  21. 

HISTOLOGY.     Klein,  p.  17  ;     Schafer,  24  ;    Dunham,  8  ;    Nichols  & 

PATHOLOGY.    Green,  p.  12;  Gibbes,  11;  Coate,  6;  Nichols  &  Vale,  21. 

SURGERY.     Park,  p.  22 ;  Dennis,  8 ;  Roberts,  24 ;  Ashhurst,  3 ;  Treves,  29 ; 
Cheyne  &  Burghard,  5  ;  Gallaudet,  11. 

SURGERY— OPERATIVE.    Stimson,  p.  27  ;  Smith,  26  ;  Treves,  29. 

SURGERY— ORTHOPEDIC.    Young,  p.  31. 

SURGERY— MINOR.    Wharton,  p.  30.  [Wippern  & 

FRACTURES  and  DISLOCATIONS.   Stimson,  p.  27.  [Ballenger,31 

OPHTH  ALMOLOGY.    Norris  &  Oliver,  p.  21 ;  Nettleship,  21 ;  Juler,  17; 

OTOLOGY.  Politzer,  p.  23;  Burnett,  5;  Field,  10;  Bacon,  3. 

LARYNGOLOGY  and  RHINOLOGY.  Coakley,  p.  6  ; 

DENTISTRY.    Essig  (Prosthetic),  p.  9  ;  Kirk  (Operative),  17  ;  Ameri- 
can System.  2  ;  Coleman,  6;  Burchard,  4. 

URINARY  DISEASES.    Roberts,  p.  24  ;  Black,  4. 

VENEREAL    DISEASES.      Taylor,  p.  28  ;    Hayden,  14  ;    Cornil,  6  ; 

SEXUAL  DISORDERS.    Fuller,  p.  11  ;  Taylor,  28. 

DERMATOLOGY.     Hyde,  p.  16  ;  Jackson,  16  ;  Pye-Smith,  23  ;  Mor- 
ris, 20 ;  Jamieson,  16 ;  Hardaway,  12  ;  Grindon,  12. 

GYNECOLOGY.      American  System,  p.  3 ;    Thomas   &  Munde",  29 
Emmet,  9  ;  Davenport,  7  ;  May,  19  ;  Dudley,  8  ;  Crockett,  7. 

OBSTETRICS.     American  System,  p.  3  ;   Davis,  7  ;   Parvin,  22  ;   Play- 
fair.  22  ;  King,  17  ;  Jewett,  16  ;  Evans,  9. 

PEDIATRICS.    Smith,  p.  26  ;  Thomson,  29  ;  Williams,  31  ;  Tuttle,  30. 

HYGIENE.     Egbert,  p.  9  ;  Richardson,  24  ;  Coates,  6. 

MEDICAL  JURISPRUDENCE.    Taylor,  p.  28. 

QUIZ  SERIES,  POCKET  TEXT-BOOKS  and  MANUALS. 

Pp.  17,  25  and  27. 
7.15.00. 


2       LEA  BROTHERS  &  Co.,  PHILADELPHIA  AND  NEW  YORK. 


ABBOTT  (A.  C.).  PRINCIPLES  OF  BACTERIOLOGY:  a  Practical 
Manual  for  Students  and  Physicians.  New  (5th)  edition  thoroughly 
revised  and  greatly  enlarged.  In  one  handsome  12mo.  vol.  of  585  pages, 
with  109  engrav. ,  of  which  26  are  colored .  Cloth,  $2.75,  net. 


One  of  its  most  attractive  charac- 
teristics is  that  the  directions  are  so 
clearly  given  that  anyone  with  a 
moderate  amount  of  laboratory  train- 
ing can,  with  a  little  care  as  to 
detail,  make  his  experiments  suc- 
AMEBICAN  SYSTEM  OF  PRACTICAL  MEDICINE. 


cessfully.  To  those  who  require  a 
condensed  yet  nevertheless  complete 
work  upon  Bacteriology  we  most 
cordially  recommend  it. —  The  Thera- 
peutic Gazette. 


American  Authors.    Edited  In 


ASYS- 
ons  by  T~ 

ALFRED  L.  LOOMIS,  M.D.,  LL.D., 
In  four  very  handsome  octavo 

volumes  of  about  900  pages  each,  fully  illustrated.  Complete  work 
now  ready.  Per  volume,  cloth,  $5 ;  leather,  $6 ;  half  Morocco,  $7. 
For  tale  by  subscription  only.  Prospectus  free  on  application. 


TEM  OF  PRACTICAL  MEDICINE.    In  contributions  by  Various 

I  by  Ai 
and  W.  OILMAN  THOMPSON,  M.  D. 


Every  chapter  is  a  masterpiece  of 
completeness,  and  is  particularly  ex- 
cellent in  regard  to  treatment,  many 
original  prescriptions,  formulae, 
charts  and  tables  being  given  for  the 
guidance  of  the  practitioner. 

"The  American  Svstem  of  Medi- 


cine" is  a  work  of  which  every 
American  physician  may  reasonably 
feel  proud,  and  in  which  every  prac- 
titioner will  find  a  safe  and  trust- 
worthy counsellor  in  the  daily  re- 
sponsibilities of  practice. — The  Ohio 
Medical  Journal. 


AMERICAN  SYSTEM  OP  DENTISTRY.  In  treatises  by  various 
authors.  Edited  by  WILBUR  F.  LITCH,  M.D.,  D.D.S.  In  three  very 
handsome  super-royal  octavo  volumes,  containing  about  3200  pages, 
with  1873  illustrations  and  many  full-page  plates.  Per  vol.,  cloth, 
$6;  leather,  $7 ;  half  Morocco,  $8.  For  sale  by  subscription  only.  Pros- 
pectus free  on  application  to  the  Publishers. 

AMERICAN  TEXT-BOOKS  OP  DENTISTRY.  In  Contribu- 
tions by  Eminent  American  Authorities.  In  two  very  handsome 
octavo  volumes,  richly  illustrated  : 

PROSTHETIC  DENTISTRY.  Edited  by  CHARLES  J.  ESSIG,  M.D., 
D.D.S.,  Professor  of  Mechanical  Dentistry  and  Metallurgy,  Department 
of  Dentistry,  University  of  Pennsylvania,  Philadelphia.  760  pages, 
983  engravings.  Cloth,  $6 ;  leather,  $7.  Net. 


No  more  thorough  production  will 
be  found  either  in  this  country  or  in 
any  country  where  dentistry  is  un- 
derstood as  a  part  of  civilisation. — 
The  International  Dental  Journal. 


It  is  up  to  date  in  every  particular. 
It  is  a  practical  course  on  prosthetics 
which  any  student  can  take  up  dur- 
ing or  after  college. — Dominion  Den- 
tal Journal. 

OPERATIVE  DENTISTRY.  Edited  by  EDWARD  C.  KIRK,  D.D.S. , 
Professor  of  Clinical  Dentistry,  Department  of  Dentistry,  University 
of  Pennsylvania.  699  pages,  751  engravings.  Cloth,  $5.50 ;  leather, 
$6.50.  Net. 


Written  by  a  number  of  practi- 
tioners as  well  known  at  the  chair 
as  in  journalistic  literature,  many  of 
them  teachers  of  eminence  in  our 
colleges.  It  should  be  included  in 
the  list  of  text-books  set  down  as 
most  useful  to  the  college  student. — 
The  Dental  News. 


It  is  replete  in  every  particular 
and  treats  the  subject  in  a  progressive 
manner.  It  is  a  book  that  every 
progressive  dentist  should  possess, 
and  we  can  heartily  recommend  it 
to  the  profession.— The  Ohio  Dental 
Journal. 


LEA  BROTHERS  &  Co.,  PHILADELPHIA  AND  NEW  YORK.       3 

AMERICAN  SYSTEMS  OF  GYNECOLOOY  AND  OBSTET- 
RICS. In  treatises  by  the  most  eminent  American  specialists.  Gyne- 
cology  edited  by  MATTHEW  D.  MANN,  A.  M.,  M.  D.,  and  Obstetrics 
edited  by  BARTON  C.  HIRST,  M.  D.  In  four  large  octavo  volumes 
comprising  3612  pages,  with  1092  engravings,  and  8  colored  plates.  Per 
volume,  cloth,  $5  ;  leather,  $6 ;  hair  Russia,  $7.  For  sale  by  subscrip- 
tion only.  Prospectus  free  on  application  to  the  Publishers. 

AMERICAN  TEXT-BOOK  OF  ANATOMY.     See  Gerrish,  page  11. 

ALLEN  (HARRISON).  A  SYSTEM  OF  HUMAN  ANATOMY; 
WITH  AN  INTRODUCTORY  SECTION  ON  HISTOLOGY,  by 
E.  O.  SHAKESPEARE,  M.D.  Comprising  813  double-columned  quarto 
pages,  with  380  engravings  on  stone,  109  plates,  and  241  wood  cuts 
in  the  text.  In  six  sections,  each  in  a  portfolio.  Price  per  section,  $3.50. 
Also,  bound  in  one  volume,  cloth,  $23.  Sold  by  subscription  only. 

A  PRACTICE  OF  OBSTETRICS  BY  AMERICAN  AU- 
THORS. See  Jewett,  page  17. 

A  TREATISE   ON   SURGERY  BY  AMERICAN  AUTHORS. 

FOR  STUDENTS  AND  PRACTITIONERS  OF  SURGERY  AND 
MEDICINE.    Edited  by  ROSWELL  PARK,  M.D.     See  page  22. 

ASHHURST  (JOHN,  JR.).  THE  PRINCIPLES  AND  PRACTICE 
OF  SURGERY.    For  the  use  of  Students  and  Practitioners.    Sixth 
and  revised  edition.    In  one  large  and  handsome  octavo  volume  of 
1161  pages,  with  656  engravings.  Cloth,  $6 ;  leather,  $7. 
As  a  masterly  epitome  of  what  has  I  text-book,  we  do  not  know  its  equal. 


been  said  and  done  in  surgery,  as  a 
succinct  and  logical  statement  of  the 
principles  of  the  subject,  as  a  model 


It  is  the  best  single  text-book  of 
surgery  that  we  have  yet  seen  in  this 
country. — New  York  Post- Graduate. 


A  SYSTEM  OF  PRACTICAL  MEDICINE  BY  AMERICAN 
AUTHORS.  Edited  by  WILLIAM  PEPPER,  M.  D.,  LL.  D.  In  five 
large  octavo  volumes,  containing  5573  pages  and  198  illustrations.  Price 
per  volume,  cloth,  $5 ;  leather  $6  ;  half  Russia,  $7.  Sold  by  subscrip- 
tion only.  Prospectus  free  on  application  to  the  Publishers. 

ATTFIELD  (JOHN).  CHEMISTRY ;  GENERAL,  MEDICAL  AND 
PHARMACEUTICAL.  New  (16th)  edition,  specially  revised  by  the 
Author  for  America.  In  one  handsome  12mo.  volume  of  784  pages, 
with  88  illustrations.  Cloth,  $2.50,  net. 

It  is  replete  with  the  latest  inform-  been  adopted,  bringing  the  work  into 
ation,  and  considers  the  chemistry  of  close  touch  with  the  latest  United 
every  substance  recognized  officially  States  Pharmacopoeia,  of  which  it  is 
or  in  general  practice.  The  modern  a  worthy  companion. — ThePittsburg 
scientific  chemical  nomenclature  has  i  Medical  Review. 

BARNES  (ROBERT  AND  FANCOURT).  A  SYSTEM  OF  OB- 
STETRIC MEDICINE  AND  SURGERY.  Octavo,  872  pages,  with 
231  illus.  Cloth,  $5  :  leather,  $6. 

BACON  (GORHAM).    ON  THE  EAR.   One  12mo.  volume,  400  pages, 

109  engravings  and  a  colored  plate.     Cloth,  net,  $2.00.   • 
It  is  the  best  manual  upon  otology,    dents  of  medicine — Cleveland  Jour- 
An  intensely  practical  book  for  stu-    nal  of  Medicine. 


4       LEA  BROTHERS  &  Co.,  PHILADELPHIA  AND  NEW  YORK. 

BARTHOLOW  (ROBERTS).  CHOLERA;  ITS  CAUSATION,  PRE- 
VENTION AND  TREATMENT.  In  one  12mo.  volume  of  127  pages, 
with  9  illustrations.  Cloth,  $1.25. 

BILLINGS  (JOHN  S.).  THE  NATIONAL  MEDICAL  DICTIONARY. 
Including  in  one  alphabet  English,  French,  German,  Italian  and 
Latin  Technical  Terms  used  in  Medicine  and  the  Collateral  Sciences. 
In  two  very  handsome  imperial  octavo  volumes  containing  1574 
pages  and  two  colored  plates.  Per  volume,  cloth,  $6 ;  leather,  $7 ; 
half  Morocco,  $8.50.  For  sale  by  subscription  only.  Specimen  pages 
on  application  to  the  publishers. 

BLACK  (D.  CAMPBELL).  THE  URINE  IN  HEALTH  AND 
DISEASE,  AND  URINARY  ANALYSIS,  PHYSIOLOGICALLY 
AND  PATHOLOGICALLY  CONSIDERED.  In  one  12mo.  volume 
of  256  pages,  with  73  engravings.  Cloth,  $2.75. 


Concise,  practical,  clinical,  well 
illustrated  and  well  printed. — Mary- 
land Medical  Journal. 


A  concise,  yet  complete  manual, 
treating  of  the  subject  from  a  prac- 
tical and  clinical  standpoint. — The 
Ohio  Medical  Journal. 

BLOXAM  (C.  L.).  CHEMISTRY,  INORGANIC  AND  ORGANIC. 
With  Experiments.  New  American  from  the  fifth  London  edition. 
In  one  handsome  octavo  volume  of  727  pages,  with  292  illustrations. 
Cloth,  $2 ;  leather,  $3. 

BROCKWAY  (F.  J.).  A  POCKET  TEXT-BOOK  OF  ANATOMY. 
In  one  handsome  12mo.  volume  of  about  400  pages,  with  many  illus- 
trations. Shortly.  Lea's  Series  of  Pocket  Text-books,  edited  by  BERN 
B.  GALLAUDET,  M.  D.  See  page  18. 

BRUCE  (J.  MITCHELL).  MATERIA  MEDICA  AND  THERA- 
PEUTICS. New  (6th)  edition.  In  one  12mo.  volume  of  600  pages. 
Just  ready.  Cloth,  $1.50,  net.  See  Student's  Series  of  Manuals, 
page,  27. 


This  new  edition  increases  the 
value  and  more  firmly  establishes 
the  reputation  of  a  work  already 


is  a  good  one  for  the  student  and  as 
a  busy  man's  reference. — Medical 
Review  of  Reviews. 


known  and  appreciated.    The  book 

PRINCIPLES  OF  TREATMENT.    In  one  octavo  volume  of  625 

pages.     Cloth,  $3.75,  net.    Just  ready. 

One  of  the  most  useful  books  in    facts,  and  receive  numerous  valuable 


which  the  practitioner  can  invest. 
It  is  a  book  worthy  of  reading  from 
cover  to  cover  ;  for  if  he  does  so  with 
studious  intent,  he  will  learn  many 


suggestions  that  he  can  carry  with 
him  to  the  bedside  for  the  good  of 
his  patient. —  Virginia  Medical  Semi- 
Mouthly. 


BRYANT  (THOMAS).  THE  PRACTICE  OF  SURGERY.  Fourth 
American  from  the  fourth  English  edition.  In  one  imperial  octavo  vol. 
of  1040  pages,  with  727  illustrations.  Cloth,  $6.50;  leather,  $7.50. 

BURCHARD  (HENRY  H.).  DENTAL  PATHOLOGY  AND  THER- 
APEUTICS. Handsome  octavo,  575  pages,  with  400  illustrations. 
Cloth,  net,  $5.00;  leather,  net,  $6.00. 

In  the  treatment  of  the  subject  the  a  valuable  text-book  on  a  subject 
method  pursued  by  the  author  is  I  which  has  heretofore  not  been  ade- 
logical  and  sequential.  The  work  is  I  quately  represented. — Dentitl  (~'OH>IK>S 


LEA  BROTHERS  &  Co.,  PHILADELPHIA  AND  NEW  YORK.       5 

BURNETT  (CHARLES  H.).  THE  EAR :  ITS  ANATOMY,  PHYSI- 
OLOGY AND  DISEASES.  A  Practical  Treatise  for  the  Use  of 
Students  and  Practitioners.  Second  edition.  In  one  8vo.  volume  of 
580  pages,  with  107  illustrations.  Cloth,  $4 ;  leather,  $5. 

CARTER  (R.  BRUDENELL)  AND  FROST  (W.  ADAMS).  OPH- 
THALMIC SURGERY.  In  one  pocket-size  12mo.  volume  of  559 
pages,  with  91  engravings  and  one  plate.  Cloth,  $2.25.  See  Series  of 
Clinical  Manuals,  page  25. 

CASPARI   (CHARLES   JR.).     A  TREATISE  ON  PHARMACY. 

For  Students  and  Pharmacists.     In  one  handsome  octavo  volume  of 
680  pages,  with  288  illustrations.     Cloth,  $4.50. 

The  author's  duties  as  Professor  ;  student  who  cannot  understand  must 
of  Theory  and  Practice  of  Pharmacy  ]  be  dull  indeed.  The  book  is  full  of 
in- the  Maryland  College  of  Phar-  new,  clean,  sharp  illustrations,which 
macy,  and  his  contact  with  students  tell  the  story  frequently  at  a  glance, 
made  him  aware  of  their  exact  The  index  is  full  and  accurate. — 
wants  in  the  matter  of  a  manual.  I  National  Druggist. 
His  work  is  admirable,  and  the  | 

CHAPMAN  (HENRY  C.).  A  TREATISE  ON  HUMAN  PHYSI- 
OLOGY. New  (2d)  edition.  In  one  octavo  volume  of  921  pages, 
with  595  illustrations.  Cloth,  $4.25 ;  leather,  $5.25,  net. 


In  every  respect  the  work  fulfils 
its  promise,  whether  as  a  complete 
treatise  for  the  student  or  as  an  ad- 


mirable work  of  reference  for  the 
physician. — North  Carolina  Medical 
Journal. 


CHARLES  (T.  CRANSTOUN).  THE  ELEMENTS  OF  PHYSIO- 
LOGICAL AND  PATHOLOGICAL  CHEMISTRY.  Octavo,  451 
pages,  with  38  engravings  and  1  colored  plate.  Cloth,  $3.50. 

CHEYNE  (W.  "WATSON).  THE  TREATMENT  OF  WOUNDS, 
ULCERS  AND  ABSCESSES.  In  one  12mo.  volume  of  207  pages. 
Cloth,  $1.25. 


One  will  be  surprised  at  the 
amount  of  practical  and  useful  in- 
formation it  contains;  information 
that  the  practitioner  is  likely  to 


need  at  any  moment.  The  sections 
devoted  to  ulcers  and  abscesses  are 
indispensable  to  any  physician. — 
The  Charlotte  Medical  Journal. 


CHEYNE  (W.  W.)  AND  BURGHARD  (F.  F.).  SURGICAL 
TREATMENT.  In  seven  octavo  volumes,  illustrated.  Now  ready. 
Volume  1,  299  pages  and  66  engravings.  Cloth,  $3.00  net.  Volume  2, 
382  pages,  141  engravings.  Cloth,  $4.00  net.  Vol.  3,  305  pages,  100 
engravings.  Cloth,  $3.50,  net.  Vol.  IV.,  in  press. 


The  book  is  especially  strong  from 
the  practical  point  of  view,  and  con- 
tains many  useful  hints,  often  upon 
minor  details  which  contribute  so 
much  to  surgical  success.  Treat- 


ment receives  a  very  large  share  of 
attention.  The  illustrations  are  clear 
and  useful,  and  the  index  has  evi- 
dently been  very  carefully  made. — 
Medical  Itecord. 


CLARKE  (W.  B.)  AND  LOCKWOOD  (C.  B.).  THE  DISSECTOR'S 
MANUAL.  In  one  12mo.  volume  of  396  pages,  with  49  engravings. 
Cloth,  $1.50.  See  Students'  Series  of  Manuals,  page  27. 

CLELAND  (JOHN).  A  DIRECTORY  FOR  THE  DISSECTION  OF 
THE  HUMAN  BODY.  In  one  12mo.  vol.  of  178  pages.  Cloth,  $1.25. 

CLINICAL  MANUALS.     See  Series  of  Clinical  Manuals,  page  25. 


6      LEA  BBOTHERS  &  Co.,  PHILADELPHIA  AND  NEW  YOEK. 

CLOUSTON  (THOMAS  S.).  CLINICAL  LECTURES  ON  MENTAL 
DISEASES.  New  (5th)  edition.  In  one  octavo  volume  of  750  pages, 
with  19  colored  plates.  Cloth,  $4.25,  net. 

.gBT^FOLSOM'S  Abstract  of  Laws  of  U.  S.  on  Custody  of  Insane,  octavo, 
$1.50,  is  sold  in  conjunction  with  Clouston  on  Mental  Diseases  for 
$5.00,  net,  for  the  two  works. 

CLOWES  (FRANK).  AN  ELEMENTARY  TREATISE  ON  PRACTI- 
CAL CHEMISTRY  AND  QUALITATIVE  INORGANIC  ANALY- 
SIS. From  the  fourth  English  edition.  In  one  handsome  12mo. 
volume  of  387  pages,  with  55  engravings.  Cloth,  $2.50. 

COAKLEY  (CORNELIUS  G.).    THE  DIAGNOSIS  AND  TREAT- 
MENT   OF    DISEASES    OF    THE    NOSE,    THROAT,    NASO- 
PHARYNX AND  TRACHEA.     In  one  12mo.  volume  of  526  pages 
with  92  engravings  and  2  colored  plates.    Cloth,  $2.75.  net. 
The  work  is  a  convenient  and  in-  f  may  be  recommended  as  a  complete 
expensive  guide  to  the  entire  field  of  and  trustworthy  summary    of   the 
diseases  of  the  nose  and  throat,  which  I  subject. — Medical  News. 

COATES  (W.  E.,  JR.).  A  POCKET  TEXT-BOOK  OF  BACTE- 
RIOLOGY AND  HYGIENE.  In  one  handsome  12mo.  volume  of 
about  350  pages,  with  many  illustrations.  Shortly.  Lea's  Series  of 
Pocket  Text-books,  edited  by  BERN  B.  GALLATJDET,  M.  D.  See 
page  18. 

COATS  (JOSEPH).  A  TREATISE  ON  PATHOLOGY.  In  one  vol. 
of  829  pages,  with  339  engravings.  Cloth,  $5.50 ;  leather,  $6.50. 

COLEMAN  (ALFRED).  A  MANUAL  OF  DENTAL  SURGERY 
AND  PATHOLOGY.  With  Notes  and  Additions  to  adapt  it  to  Amer- 
ican Practice.  By  THOS.  C.  STELLWAGEN,  M.A.,  M.D.,  D.D.S.  In  one 
handsome  octavo  vol.  of  412  pages,  with  331  engravings.  Cloth,  $3.25. 

COLLINS  (C.  P.).  A  POCKET  TEXT-BOOK  OF  MEDICAL 
DIAGNOSIS.  In  one  handsome  12mo.  volume  of  about  350  pages, 
with  many  illustrations.  Shortly.  Lea's  Series  of  Pocket  Text-books, 
edited  by  BEEN  B.  GALLATTDET,  M.  D.  See  page  18. 

COLLINS  (H.  D.)  AND  ROCKWELL  (W.  H.).  A  POCKET 
TEXT-BOOK  OF  PHYSIOLOGY.  12mo.  of  316  pages,  with  153 
illustrations.  Just  ready.  Cloth,  $1.50;  flexible  red  leather,  $2.00, 
net.  Lea's  Series  of  Pocket  Text-books,  edited  by  BERN  B.  GALLAU- 
DET,  M.  D.  See  page  17. 


practitioner  with  the  advances  in 
this  subject. — The  Pliysician  and 
Surgeon. 


Well  written  and  up  to  date.  It 
is  a  manual  admirably  adapted  to 
teach  the  beginner  the  essentials  of 
physiology,  and  to  acquaint  the 

CONDIE  (D.  FRANCIS).  A  PRACTICAL  TREATISE  ON  THE  DIS- 
EASES OF  CHILDREN.  Sixth  edition,  revised  and  enlarged.  In 
one  large  8vo.  volume  of  719  pages.  Cloth,  $5.25 ;  leather,  $6.25. 

CORNIL  (V.).  SYPHILIS:  ITS  MORBID  ANATOMY,  DIAGNO- 
SIS AND  TREATMENT.  Translated,  with  Notes  and  Additions,  by 
J.  HENRY  C.  SIMES,  M.D.  and  J.  WILLIAM  WHITE,  M.  D.  In  one 
8vo.  volume  of  461  pages,  with  84  illustrations.  Cloth,  $3.75. 


LEA  BROTHERS  &  Co.,  PHILADELPHIA  AND  NEW  YORK.       7 

CROCKETT  (M.  A.).  A  POCKET  TEXT-BOOK  OF  DISEASES 
OF  WOMEN.  In  one  handsome  12mo.  volume  of  368  pages,  with 
107  illustrations.  Just  ready.  Cloth,  $1. 50,  net;  flexible  leather,  $2.00, 
net.  Lea's  Series  of  Pocket  Text-books,  edited  by  BERN  B.  GALLAU- 
DET,  M.  D.  See  page  17. 
This  is,  like  all  the  other  manuals  book  for  practitioners. — St.  Louis 

in  this  series,  a  most  excellent  guide    Medicaland  Surgical  Journal. 

for  students  and  a  handy  reference 

CROOK    (JAMES     K.)    ON    MINERAL     WATEES     OF     THE 
UNITED  STATES.   Octavo,  575  pages.   Just  ready.   Cloth,  $3.50,  net. 
In  such  a  book  as  this  the  medical  ]  of  every  water  of  any  known  medici- 
profession  will  find  a  wonderful  ally ;  !  nal    properties. — The    Louisville 
it  is  remarkably  complete  in  every  j  Monthly  Journal. 
detail,  giving  the  results  of  analyses  j 

CULBRETH  (DAVID  M.  R.).  MATERIA  MEDIC  A  AND  PHAR- 
MACOLOGY. In  one  handsome  octavo  volume  of  812  pages,  with 
445  illustrations.  Cloth,  $4.75. 

CUSHNY   (ARTHUR  R.).   TEXT-BOOK  OF  PHARMACOLOGY. 

Handsome  8vo.,  728  pages,  with  47  illus.    Cloth,  $3.75,  net. 
The  best  exposition  of  our  knowl-  !  acquainting  themselves  with  the  very 
edge  of  pharmacology  which  has  yet    latest  knowledge  on  this  very  im- 
been  given  to  the  medical  public,  j  portant  subject. — The  Montreal  Med- 
We  can  cordially  recommend  it  to    ical  Journal. 
all  our  readers  who  are  desirous  of 

DAI/TON  (JOHN  C.).   A  TREATISE  ON  HUMAN  PHYSIOLOGY. 

Seventh  edition.     Octavo,   722  pages,  with   252  engravings.     Cloth, 
$5 ;  leather,  $6. 

DOCTRINES  OF  THE  CIRCULATION  OF  THE  BLOOD.  In 


one  handsome  12mo.  volume  of  293  pages.     Cloth,  $2. 

DAVENPORT  (F.  H.).  DISEASES  OF  WOMEN.  A  Manual  of 
Gynecology.  For  the  use  of  Students  and  Practitioners.  New 
(3d)  edition.  In  one  handsome  12mo.  volume  of  387  pages,  with  150 
illustrations.  Cloth,  $1.75,  net. 


Dr.  Davenport  has  the  happy 
faculty  of  selecting  just  those  points 
in  gynecological  therapeutics  and 
surgery  which  the  student  and  junior 


knowing,  and  presents  these  princi- 
ples in  a  clear,  concise  and  thorough 
manner.  The  book  can  be  highly 
commended. —  The  Medical  Age. 


practitioner  most  stand  in  need  of 

DAVIS  (EDWARD  P.).     A  TREATISE  ON  OBSTETRICS.    FOR 

STUDENTS    AND    PRACTITIONERS.      In   one  very    handsome 

octavo  volume  of  546  pages,  with  217   engravings  and  30  full-page 

plates  in  colors  and  monochrome.    Cloth,  $5 ;  leather,  $6. 

From  a  practical  standpoint  the  j  thoroughly  scientific  and    brilliant 

work  is  all  that  could  be  desired.  A  3  treatise  on  obstetrics.  —Med.  News. 

DAVIS  (F.  H.).    LECTURES  ON  CLINICAL  MEDICINE.    Second 
edition.    In  one  12mo.  volume  of  287  pages.     Cloth,  $1.75. 

DE  LA  BECHE'S  GEOLOGICAL  OBSERVER.    In  one  large  octavo 
volume  of  700  pages,  with  300  engravings.     Cloth,  $4. 


8       LEA  BROTHERS  &  Co.,  PHILADELPHIA  AND  NEW  YORK. 

DENNIS  (FREDERIC  S.)  AND  BILLINGS  (JOHN  S.).  A  SYS- 
TEM OF  SURGERY.  In  contributions  by  American  Authors. 
Complete  work  in  four  very  handsome  octavo  volumes,  containing 
3652  pages,  with  1585  engravings  and  45  full-page  plates  in  colors 
and  monochrome.  Per  volume,  cloth,  $6.00;  leather,  $7.00;  half 
Morocco,  gilt  back  and  top,  $8.50.  For  sale  by  subscription  only. 
Full  prospectus  free  on  application  to  the  publishers. 
No  work  in  English  can  be  con- 1  American  Journal  of  the  Medical 

sidered  as  the  rival  of  this. — The  '  Sciences. 

DERCUM  (FRANCIS  X.,  EDITOR).  A  TEXT-BOOK  ON 
NERVOUS  DISEASES.  By  American  Authors.  In  one  handsome 
octavo  volume  of  1054  pages,  with  341  engravings  and  7  colored 
plates.  Cloth,  $6.00 ;  leather,  $7.00.  Net. 

The  most  thoroughly  up-to-date        The  best  text-book  in  any  Ian- 


treatise  that  we  have  on  this  subject. 


guage. — The  Medical  Fortnightly. 


— American  Journal  of  Insanity. 

DE  SCHWEINITZ  (GEORGE  E.).  THE  TOXIC  AMBLYOPIAS. 
Their  Classification,  History,  Symptoms,  Pathology  and  Treatment. 
Very  handsome  octavo,  240  pages,  46  engravings,  and  9  full-page 
plates  in  colors.  Limited  edition,  de  luxe  binding,  $4.  Net. 

DRAPER  (JOHN  C.).  MEDICAL  PHYSICS.  A  Text-book  for  Stu- 
dents and  Practitioners  of  Medicine.  In  one  handsome  octavo  volume 
of  734  pages,  with  376  engravings.  Cloth,  $4. 

DRUITT  (ROBERT).  THE  PRINCIPLES  AND  PRACTICE  OF 
MODERN  SURGERY.  A  new  American,  from  the  twelfth  London 
edition,  edited  by  STANLEY  BOYD,  F.  R.  C.  S.  In  one  large  octavo 
volume  of  965  pages,  with  373  engravings.  Cloth,  $4 ;  leather,  $5. 

DUANE  (ALEXANDER).  A  DICTIONARY  OF  MEDICINE  AND 
THE  ALLIED  SCIENCES.  Comprising  the  Pronunciation,  Deriva- 
tion and  Full  Explanation  of  Medical,  Dental,  Pharmaceutical  and 
Veterinary  Terms.  Together  with  much  Collateral  Descriptive  Mat- 
ter. Numerous  Tables,  etc.  New  (3d)  edition.  Square  octavo  of  652 
pages,  with  8  colored  plates.  Just  ready.  Cloth,  $3.00,  net;  limp 
leather,  $4.00,  net. 

DUDLEY  (E.  C.).  THE  PRINCIPLES  AND  PRACTICE  OF 
GYNECOLOGY.  New  (2d)  edition.  Handsome  octavo  of  717  pages, 
with  453  illustrations  in  black  and  colors,  and  8  colored  plates.  Cloth, 
$5.00,  net;  leather,  $6.00,  net;  half  Morocco,  $6.50,  net.  Just  ready. 


The  book   can  be  safely    recom- 
mended as  a  complete  and  reliable 


tice  of  modern  gynecology. — Inter- 
national Medical  Magazine. 


exposition  of  the  principles  and  prac- 

DUNCAN  (J.  MATTHEWS).  CLINICAL  LECTURES  ON  THE 
DISEASES  OF  WOMEN.  Delivered  in  St.  Bartholomew's  Hospital. 
In  one  octavo  volume  of  175  pages.  Cloth,  $1.50. 

DUNHAM  (EDWARD    K.).      MORBID    AND    NORMAL     HIS- 
TOLOGY.   Octavo,  450  pages,with  363  illustrations.  Cloth,  $3.25,  net. 
The  best  one- volume  text  or  refer- 1  of  published  in  America.—  Virginia 
ence  book  on  histology  that  we  know  I  Medical  Semi- Monthly. 

NORMAL  HISTOLOGY.    New  (2d)  edition.    Octavo,  319  pages, 

with  244  illustrations.    Just  ready.     Cloth,  $2.50,  net. 


LEA  BROTHERS  &  Co.,  PHILADELPHIA  AND  NEW  YORK.       9 

DUNGLISON  (ROBLEY).  A  DICTIONARY  OF  MEDICAL  SCI- 
ENCE. Containing  a  full  explanation  of  the  various  subjects  and 
terms  of  Anatomy,  Physiology,  Medical  Chemistry,  Pharmacy,  Phar- 
macology, Therapeutics,  Medicine,  Hygiene,  Dietetics,  Pathology,  Sur- 
gery, Ophthalmology,  Otology,  Laryngology,  Dermatology,  Gynecol- 
ogy,  Obstetrics,  Pediatrics,  Medical  Jurisprudence,  Dentistry,  etc.,  etc. 
By  ROBLEY  DUNGLISON,  M.  D.,  LL.  D.,  late  Professor  of  Institutes 
of  Medicine  in  the  Jefferson  Medical  College  of  Philadelphia.  Edited 
by  RICHARD  J.  DUNGLISON,  A.  M.,  M.  D.  Twenty-second  edition,  thor- 
oughly revised  and  greatly  enlarged  and  improved,  with  the  Pronuncia- 
tion, Accentuation  and  Derivation  of  the  Terms.  With  Appendix. 
In  one  magnificent  imperial  octavo  volume  of  about  1400  pages. 
Shortly.  Notices  of  previous  edition  are  appended. 


scarcely  be  measured. — Ned.  Record. 

Pronunciation  is  indicated  by  the 

phonetic  system.  The  definitions  are 


The  most  satisfactory  and  authori- 
tative guide  to  the  derivation,  defini- 
tion and  pronunciation   of  medical      lf.^^^v^a  ^^^    -^~~  w,uu*.,»,Ut,  ~,+  ~ 
terms.— The  Charlotte Med.  Journal,    {^usually' clear  and  TondseT"fhe 

Covering  the  entire  field  of  medi-  j  book  is  wholly   satisfactory. —  Uni- 
cine,    surgery    and    the     collateral  I  versity  Medical  Magazine. 
sciences,  its  range  of  usefulness  can 

EDES  (ROBERT  T.).  TEXT-BOOK  OF  THERAPEUTICS  AND 
MATERIA  MEDICA.  In  one  8vo.  volume  of  544  pages.  Cloth,  $3.50  ; 
leather,  $4.50. 

EDIS  (ARTHUR  W.).  DISEASES  OF  WOMEN.  A  Manual  for 
Students  and  Practitioners.  In  one  handsome  8vo.  volume  of  576  pages, 
with  148  engravings.  Cloth,  $3 ;  leather,  $4. 

EGBERT  (SENECA).  A  MANUAL  OF  HYGIENE  AND  SANI- 
TATION. In  one  12mo.  volume  of  359  pages,  with  63  illustrations. 
Cloth,  Net,  $2.25. 


It  is  written  in  plain  language, 
and,  while  primarily  designed  for 
physicians,  it  can  be  studied  with 
profit  by  any  one  of  ordinary  intel- 


ligence. The  writer  has  adapted  it 
to  American  conditions,  and  his 
suggestions  are,  above  all,  practical. 
—  The  New  York  Medical  Journal. 


ELLIS  (GEORGE  V1NER).  DEMONSTRATIONS  IN  ANATOMY. 
Eighth  edition.  Octavo,  716  pages,  with  249  engravings.  Cloth, 
$4.25 ;  leather,  $5.25. 

EMMET  (THOMAS  ADDIS).  THE  PRINCIPLES  AND  PRAC- 
TICE OF  GYNAECOLOGY.  Third  edition.  Octavo,  880  pages,  with 
150  original  engravings.  Cloth,  $5 ;  leather,  $6. 

ERICHSEN  (JOHN  E.).  THE  SCIENCE  AND  ART  OF  SUR- 
GERY. Eighth  edition.  In  two  large  octavo  volumes  containing 
2316  pages,  with  984  engravings.  Cloth,  $9 ;  leather,  $11. 

ESSIG  (CHARLES  J.).  PROSTHETIC  DENTISTRY.  See  American 
Text- Books  of  Dentistry,  page  2. 

EVANS  (DAVID  J.).    A  POCKET  TEXT-BOOK  OF  OBSTETRICS. 

In  one  handsome  12mo.  volume  of  about  300  pages,  with  many  illustra- 
tions. Shortly.  Lea's  Series  of  Pocket  Text-books,  edited  by  BERN  B. 
GALLAITDET/M.  D.  See  page  18. 

FARQUHARSON  (ROBERT).  A  GUIDE  TO  THERAPEUTICS. 
Fourth  American  from  fourth  English  edition,  revised  by  FRANK 
WOODBURY,  M.  D.  In  one  12mo.  volume  of  581  pages.  Cloth,  $2.50. 


10     LEA  BROTHERS  &  Co.,  PHILADELPHIA  AND  NEW  YORK. 

FIELD  (GEORGE  P.).  A  MANUAL  OF  DISEASES  OF  THE 
EAR.  Fourth  edition.  In  one  octavo  volume  of  391  pages,  with  73 
engravings  and  21  colored  plates.  Cloth,  $3.75. 

FLINT  (AUSTIN).  A  TREATISE  ON  THE  PRINCIPLES  AND 
PRACTICE  OF  MEDICINE.  Seventh  edition,  thoroughly  revised 
by  FREDERICK  P.  HENRY,  M.  D.  In  one  large  8vo.  volume  of  1143 
pages,  with  engravings.  Cloth,  $5.00;  leather,  $6.00. 


The  work  has  well  earned  its  lead- 
ing place  in  medical  literature. — 
Medical  Record. 


The  best  of  American  text-books 
on  Practice.— A mer. Medico-Surgical 
Bulletin. 

A   MANUAL   OF  AUSCULTATION  AND  PERCUSSION ;  of 

the  Physical  Diagnosis  of  Diseases  of  the  Lungs  and  Heart,  and  of 
Thoracic  Aneurism.  Fifth  edition,  revised  by  JAMES  C.  WILSON,  M.  D. 
In  one  handsome  12mo.  volume  of  274  pages,  with  12  engravings. 

A    PRACTICAL    TREATISE    ON    THE    DIAGNOSIS    AND 

TREATMENT  OF  DISEASES  OF  THE  HEART.  Second  edition 
enlarged.  In  one  octavo  volume  of  550  pages.  Cloth,  $4. 

ON  PHTHISIS :  ITS  MORBID  ANATOMY  ETIOLOGY,  ETC. 

A  Series  of  Clinical  Lectures.  In  one  8vo.  volume  of  442  pages. 
Cloth,  $3.50. 

FOLSOM  (C.  F.).  AN  ABSTRACT  OF  STATUTES  OF  U.  S. 
ON  CUSTODY  OF  THE  INSANE.  In  one  8vo.  vol.  of  108  pages. 
Cloth,  $1.50.  With  Clouston  on  Mental  Diseases  (new  edition,  see 
page  6)  $5.00,  net,  for  the  two  works. 

FORMULARY,  POCKET,  see  page  32. 

FOSTER  (MICHAEL,).  A  TEXT-BOOK  OF  PHYSIOLOGY.  New 
(6th)  and  revised  American  from  the  sixth  English  edition.  In  one 
large  octavo  volume  of  923  pages,  with  257  illustrations.  Cloth,  $4.50 ; 
leather,  $5.50. 


Unquestionably  the  best  book  that 
can  be  placed  in  the  student's  hands, 
and  as  a  work  of  reference  for  the 
busy  physician  it  can  scarcely  be 
excelled. — The  Ptiila.  Poly  clinic. 


This  single  volume  contains  all 
that  will  be  necessary  in  a  college 
course,  and  all  that  the  physician 
will  need  as  well.: — Dominion  Med. 
Monthly. 


FOTHERGILLi  (J.  MILNER).  THE  PRACTITIONER'S  HAND- 
BOOK OF  TREATMENT.  Third  edition.  In  one  handsome  octavo 
volume  of  664  pages.  Cloth,  $3.75 ;  leather,  $4.75. 

FOWNES  (GEORGE).  A  MANUAL  OF  ELEMENTARY  CHEM- 
ISTRY (INORGANIC  AND  ORGANIC).  Twelfth  edition.  Em- 
bodying WATTS'  Physical  and  Inorganic  Chemistry.  In  one  royal 
12mo.  volume  of  1061  pages,  with  168  engravings,  and  1  colored 
plate.  Cloth,  $2.75 ;  leather,  $3.25. 

FRANKLAND  (E.)  AND  JAPP  (F.R.).  INORGANIC  CHEMISTRY. 
In  one  handsome  octavo  volume  of  677  pages,  with  51  engravings  and 
2  plates.  Cloth,  $3.75 ;  leather,  $4.75. 

FULLER  (HENRY).  ON  DISEASES  OF  THE  LUNGS  AND  AIR 
PASSAGES.  Their  Pathology,  Physical  Diagnosis,  Symptoms  and 
Treatment.  From  second  English  edition.  In  one  8vo.  volume  of  475 
pages.  Cloth,  $3.50. 


LEA  BROTHERS  &  Co.,  PHILADELPHIA  AND  NEW  YORK.     11 


FULLER  (EUGENE).  DISORDERS  OF  THE  SEXUAL  OR- 
GANS IN  THE  MALE.  In  one  very  handsome  octavo  volume  of 
238  pages,  with  25  engravings  and  8  full-page  plates.  Cloth,  $2. 


It  is  an  interesting  work,  and  one 


which  is  timely  and  needed. — Medi- 
cal Fortnightly. 

The  book  is  valuable  and  instruc- 
tive and  brings  views  of  sound 
GALLiAUDET  (BERN  B.).  A 


pathology  and  rational  treatment  to 


many  cases  of  sexual  disturbance 
whose  treatment  has  been  too  often 
fruitless  for  good.  —  Annals  of 
Surgery. 

POCKET  TEXT-BOOK  ON  SUR- 
GERY. In  one  handsome  12mo.  volume  of  about  400  pages,  with  many 
illustrations.  Shortly.  Lea's  Series  of  Pocket  Text-books,  edited  by 
BERN  B.  GALLATJDET,  M.  D.  See  page  17. 

GANT  (FREDERICK  JAMES).  THE  STUDENT'S  SURGERY.  A 
Multum  in  Parvo.  In  one  square  octavo  volume  of  845  pages,  with 
159  engravings.  Cloth,  $3.75. 

GERRISH  (FREDERIC  H.).  A  TEXT-BOOK  OF  ANATOMY. 
By  American  Authors.  Edited  by  Frederic  H.  Gerrish,  M.  D.  In  one 
imp.  octavo  volume  of  915  pages,  with  950  illustrations  in  black  and 
colors.  Cloth,  $6.50;  flexible  waterproof,  $7;  leather,  $7.50,  net; 
half  Morocco,  $8.00,  net. 

any  similar  text-book  with  which 
we  are  familiar. — The  Boston  Medi- 
cal and  Surgical  Journal. 

We  believe  that  this  volume  not 
only  takes  rank  with  all  other  works 
on  anatomy,  but  in  some  respects  is 
superior  to  any  now  available. — The 
Chicago  Medical  Recorder. 


The  illustrations  far  outnumber 
ani  exceed  in  size  and  in  profusion 
of  colors  those  in  any  previous  work  ; 
and  they  can  well  claim  to  be  the 
most  successful  series  of  anatomical 
pictures  in  the  world. — The  Ameri- 
can Practitioner  and  News. 


The  chief  merit  in  the  book  will 
be  found  in  the  descriptive  text, 
which  is  accurate,  concise,  and  gives 
the  essentials  of  descriptive  anatomy 
with  less  waste  of  words  and  better 
emphasis  of  important  points  than 


There  is  nothing  with  which  to 
find  fault,  everything  to  praise.  The 
work  is  the  most  remarkable  and 
most  valuable  volume  of  the  year. — 
Buffalo  Medical  Journal. 


GIBBES  (HENEAGE).  PRACTICAL  PATHOLOGY  AND  MORBID 

HISTOLOGY.   Octavo,  314  pages,  with  60  illustrations.    Cloth,  $2.75. 

GRAY  (HENRY).    ANATOMY,  DESCRIPTIVE  AND  SURGICAL. 

New  and  thoroughly  revised  American  edition,  much  enlarged  in  text, 
and  in  engravings  in  black  and  colors.  In  one  imperial  octavo  volume 
of  1239  pages,  with  772  large  and  elaborate  engravings  on  wood.  Price 
of  edition  with  illustrations  in  colors  :  cloth,  $7 ;  leather,  $8.  Price 
of  edition  with  illustrations  in  black :  cloth,  $6 ;  leather,  $7. 


This  is  the  best  single  volume 
upon  Anatomy  in  the  English 
language. —  University  Medical  Mag- 
azine. 

Gray's  Anatomy  affords  the  student 
more  satisfaction  than  any  other 
treatise  with  which  we  are  familiar. 
— Buffalo  Med.  Journal. 

The  most  largely  used  anatomical 
text-book  published  in  the  English 
language. — Annals  of  Surgery. 

Particular  stress  is  laid  upon  the 
practical  side  of  anatomical  teach- 


ing, and  especially  the  Surgical 
Anatomy. — Chicago  Med.  Recorder. 

Holds  first  place  in  the  esteem  of 
both  teachers  and  students. — The 
Brooklyn  Medical  Journal. 

The  foremost  of  all  medical  text- 
books.— Medical  Fortnightly. 

Gray's  Anatomy  should  be  the 
first  work  which  a  medical  student 
should  purchase,  nor  should  he  be 
without  a  copy  throughout  his  pro- 
fessional career. — Pittsburg  Medical 
Review. 


12      LEA  BROTHERS  &  Co.,  PHILADELPHIA  AND  NEW  YORK. 

GOULD  (A.  PEARCE).  SURGICAL  DIAGNOSIS.  In  one  12mo. 
vol.  of  589  pages.  Cloth,  $2.  See  Student's  Series  of  Manuals,  p.  27. 

GREEN  (T.  HENRY).  PATHOLOGY  AND  MORBID  ANATOMY 
New  (8th)  American  from  the  eighth  London  edition.  In  one  hand- 
some octavo  volume  of  582  pages,  with  216  engravings  and  a  colored 
plate.  Cloth,  $2.50,  net. 

A  work  that  is  the  text-book  of  j  The  work  is  an  essential  to  the 
probably  four-fifths  of  all  the  stu-  practitioner — whether  as  surgeon  or 
dents  of  pathology  in  the  United  i  physician.  It  is  the  best  of  up-to- 
States  and  Great  Britain. — The  j  date  text-books. —  Virginia  Medical 
American  Practitioner  and  News.  \  Monthly. 

GREENE  (WILLIAM  H.).  A  MANUAL  OF  MEDICAL  CHEM- 
ISTRY. For  the  Use  of  Students.  Based  upon  BOWMAN'S  Medical 
Chemistry.  In  one  12mo.  vol.  of  310  pages,  with  74  illus.  Cloth,  $1.75. 

GROSS  (SAMUEL,  D.).  A  PRACTICAL  TREAT.ISE  ON  THE  DIS- 
EASES, INJURIES  AND  MALFORMATIONS  OF  THE  URINARY 
BLADDER,  THE  PROSTATE  GLAND  AND  THE  URETHRA. 
Third  edition.  Octavo,  574  pages,  with  170  illustrations  Cloth,  $4.50. 

GRINDON  (JOSEPH).  A  POCKET  TEXT-BOOK  OF  SKIN 
DISEASES.  In  one  handsome  12mo.  volume  of  350  pages,  with 
many  illustrations.  Shortly.  Lea's  Series  of  Pocket  Text-books,  edited 
by  BERN  B.  GALLAUDET,  M.  D.  See  page  18. 

HABERSHON  (S.  O.).  ON  THE  DISEASES  OF  THE  ABDOMEN 
Second  American  from  the  third  English  edition.  In  one  octavo  vol- 
ume of  554  pages,  with  11  engravings.  Cloth,  $3.50. 

HALL   (WINFIELD  S.).  TEXT-BOOK  OF  PHYSIOLOGY.  Octavo 
of  672  pages,  with  343  engravings,  and  6  full  page  colored  plates. 
Cloth,  $4.00  ;  leather,  $5.00,  net. 
Truly  a  scientific  treatment  of  the  j  of  which  needs  to  be  more  strongly 

subject".    The  clearness  with  which    impressed  upon  students     A  book 


which  makes  this  so  easily  possible 
is  to  be  highly  commended. —  West- 
ern Medical  Review. 


physiological  facts  are  demonstrated 
makes  it  of  special  value  to  the 
medical  student.  The  science  of 
physiology  is  one,  the  importance 

HAMILTON  (ALLAN  MCLANE).  NERVOUS  DISEASES,  THEIR 
DESCRIPTION  AND  TREATMENT.  Second  and  revised  edition. 
In  one  octavo  volume  of  598  pages,  with  72  engravings.  Cloth,  $4. 

HARD  A  WAY  (W.  A.).  MANUAL  OF  SKIN  DISEASES.  New  (2d) 
edition.  In  one  12mo.  volume  of  560  pages,  with  40  illustrations  and 
2  plates.  Cloth,  $2.25,  net. 

The  best  of  all  the  small  books  to  |  day  clinical  experience.     His  great 
recommend  to  students  and  practi- 
tioners.    Probably   no  one   of   our 
dermatologists  has  had  a  wider  every- 

HARE  (HOBART  AMORY)  ON  THE  MEDICAL  COMPLICA- 
TIONS AND  SEQUELS  OF  TYPHOID  FEVER.  Octavo,  276 
pages,  21  engravings  and  two  full-page  plates.  Cloth,  $2.40,  net. 


strength  is  in  diagnosis,  descriptions 
of  lesions  and  especially  in  treat- 
ment.— Indiana  Medical  Journal. 


A  very  valuable  production.  One 
of  the  very  best  products  of  Dr. 
Hare  and  one  that  every  man  can 


read  with  great  profit. —  Cleveland 
Journal  of  Medicine. 


LEA  BROTHERS  <&  Co.,  PHILADELPHIA  AND  NEW  YORK.     13 


HARE  (HOBART  AMORY).  PRACTICAL  DIAGNOSIS.  THE 
USE  OF  SYMPTOMS  IN  THE  DIAGNOSIS  OF  DISEASE.  New 
(4th)  edition.  In  one  octavo  volume  of  623  pages,  with  205  engravings 
and  14  full-page  colored  plates.  Cloth,  $5.00,  net  ;  half  Morocco, 
$6.50,  net. 


It  is  unique  in  many  respects,  and 
the  author  has  introduced  radical 
changes  which  will  be  welcomed  by 
all.  Anyone  who  reads  this  book 
will  become  a  more  acute  observer, 
will  pay  more  attention  to  the  simple 
yet  indicative  signs  of  disease,  and 


he  will  become  a  better  diagnosti- 
cian. This  is  a  companion  to  Prac- 
tical Therapeutics,  by  the  same 
author,  and  it  is  difficult  to  conceive 
of  any  two  works  of  greater  practical 
utility. — Medical  Review. 


HARE  (HOBART  AMORY).    A   TEXT-BOOK  OF  PRACTICAL 

THERAPEUTICS,  with  Special  Reference  to  the  Application  of  Reme- 
dial Measures  to  Disease  and  their  Employment  upon  a  Rational 
Basis.  With  articles  on  various  subjects  by  well-known  specialists. 
New  (8th)  and  revised  edition.  In  one  octavo  volume  of  796  pages, 
with  37  engravings  and  3  colored  plates.  Cloth,  $4.00,  net;  leather, 
$5.00,  net;  half  Morocco,  $5.50,  net.  Just  ready. 


Its  classifications  are  inimitable, 
and  the  readiness  with  which  any- 
thing can  be  found  is  the  most  won- 
derful achievement  of  the  art  of  in- 
dexing. This  edition  takes  in  all 
the  latest  discovered  remedies. — 
The  St.  Louis  Clinique. 

The  great  value  of  the  work  lies 
in  the  fact  that  precise  indications 
for  administration  are  given.  A 
complete  index  of  diseases  and 
remedies  makes  it  an  easy  reference 
work.  It  has  been  arranged  so  that 


it  can  be  readily  used  in  connection 
with  Hare's  Practical  Diagnosis. 
For  the  needs  of  the  student  and 
general  practitioner  it  has  no  equal. 
— Medical  Sentinel. 

The  best  planned  therapeutic  work 
of  the  century. — American  Prac- 
titioner and  News. 

It  is  a  book  precisely  adapted  to 
the  needs  of  the  busy  practitioner, 
who  can  rely  upon  finding  exactly 
what  he  needs. — The  National  Med- 
ical Review. 


HARE  (HOBART  AMORY,  EDITOR).  A  SYSTEM  OF  PRAC- 
TICAL THERAPEUTICS.  In  a  series  of  contributions  by  eminent 
practitioners.  In  four  large  octavo  volumes  comprising  about  4500 
pages,with  about  550  engravings.  Vol.  IV.,  now  ready.  For  sale  by  sub- 
scription only.  Full  prospectus  free  on  application  to  the  Publishers. 
Regular  price,  Vol.  IV.,  cloth,  $6 ;  leather,  $7 ;  half  Russia,  $8. 
Price  Vol.  IV.  to  former  or  new  subscribers  to  complete  work,  cloth, 
$5  ;  leather,  $6 ;  half  Russia,  $7.  Complete  work,  cloth,  $20;  leather, 
$24 ;  half  Russia,  $28. 

The  great  value  of  Hare's  System  of  Practical  Therapeutics  has  led  to  a 
widespread  demand  for  a  new  volume  to  represent  advances  in  treatment 
made  since  the  publication  of  the  first  three.  More  than  fulfilling  this 
request  the  Editor  has  secured  contributions  from  practically  a  new  corps 
of  equally  eminent  authors,  so  that  entirely  fresh  and  original  matter  is 
ensured.  The  plan  of  the  work,  which  proved  so  successful,  has  been  fol- 
lowed in  this  new  volume,  which  will  be  found  to  present  the  latest  devel- 
opments and  applications  of  this  most  practical  branch  of  the  medical  art. 
The  entire  System  is  an  unrivalled  encyclopaedia  on  the  practical  parts  of 
medicine,  and  merits  the  great  success  it  has  won  for  that  reason. 


14     LEA  BROTHERS  &  Co.,  PHILADELPHIA  AND  NEW  YORK. 

HARTSHOBNB  (HENRY).  ESSENTIALS  OF  THE  PRINCIPLES 
AND  PRACTICE  OF  MEDICINE.  Fifth  edition.  In  one  12mo. 
volume,  669  pages,  with  144  engravings.  Cloth,  $2.75 . 

A  HANDBOOK  OF  ANATOMY  AND  PHYSIOLOGY.    In  one 


12mo.  volume  of  310  pages,  with  220  engravings.     Cloth,  $1.75. 

A  CONSPECTUS  OF  THE  MEDICAL  SCIENCES.    Comprising 

Manuals  of  Anatomy,  Physiology,  Chemistry,  Materia  Medica,  Prac- 
tice of  Medicine,  Surgery  and  Obstetrics.  Second  edition.  In  one  royal 
12mo.  vol.  of  1028  pages,  with  477  illus.  Cloth,  $4.25 ;  leather,  $5. 

HAYDEN  (JAMES  R.).  A  MANUAL  OF  VENEREAL  DISEASES. 
New  (2d)  edition.  In  one  12mo.  volume  of  304  pages,  with  54  en- 
gravings. Cloth,  $1.50,  net. 


It  is  practical,  concise,  definite 
and  of  sufficient  fulness  to  be  satis- 
factory.— Chicago  Clinical  Review. 


It  is  well  written,  up  to  date,  and 
will  be  found  very  useful. — Inter- 
national Medical  Magazine. 


HAYEM  (GEORGES)  AND  HARE  (H.  A.).  PHYSICAL  AND 
NATURAL  THERAPEUTICS.  The  Remedial  Use  of  Heat,  Elec- 
tricity, Modifications  of  Atmospheric  Pressure,  Climates  and  Mineral 
Waters.  Edited  by  Prof.  H.  A.  HAKE,  M.  D.  In  one  octavo  volume 
of  414  pages,with  113  engravings.  Cloth,  $3. 


This  well-timed  volume  is  particu- 
larly adapted  to  the  requirements 
of  the  general  practitioner.  The 
section  on  mineral  waters  is  most 
scientific  and  practical.  Some  200 
pages  are  given  up  to  electricity  and 
evidently  embody  the  latest  scien- 


tific information  on  the  subject. 
Altogether  this  work  is  the  clearest 
and  most  practical  aid  to  the  study 
of  nature's  therapeutics  that  has  yet 
come  under  our  observation. — The 
Medical  Fortnightly. 


HERMAN  (G.  ERNEST).  FIRST  LINES  IN  MIDWIFERY.  In 
one  12mo.  vol.  of  198  pages,  with  80  engravings.  Cloth,  $1.25.  See 
Student's  Series  of  Manuals,  page  27. 

HERMANN  (Ij.).  EXPERIMENTAL  PHARMACOLOGY.  A  Hand- 
book of  the  Methods  for  Determining  the  Physiological  Actions  of 
Drugs.  Translated  by  ROBERT  MEADE  SMITH,  M.  D.  In  one  12mo. 
volume  of  199  pages,  with  32  engravings.  Cloth,  $1.50. 

HERRICK  (JAMES  B.).  A  HANDBOOK  OF  DIAGNOSIS.  In 
one  handsome  12mo.  volume  of  429  pages,  with  80  engravings  and  2 
colored  plates.  Cloth,  $2.50. 


We  commend  the  book  not  only  to 
the  undergraduate,  but  also  to  the 
physician  who  desires  a  ready  means 
of  refreshing  his  knowledge  of  diag- 
nosis in  the  exigencies  of  professional 
life.— Memphis  Medical  Monthly. 


Excellently  arranged,  practical, 
concise,  up-to-date,  and  eminently 
well  fitted  for  the  use  of  the  prac- 
titioner as  well  as  of  the  student. — 
Chicago  Med.  Recorder. 


HILL  (BERKELEY).    SYPHILIS  AND  LOCAL    CONTAGIOUS 
DISORDERS.    In  one  8vo.  volume  of  479  pages.    Cloth,  $3.25. 


LEA  BROTHERS  &  Co.,  PHILADELPHIA  AND  NEW  YORK.     15 

HIL.MER  (THOMAS).  A  HANDBOOK  OF  SKIN  DISEASES. 
Second  edition.  In  one  royal  12mo.  volume  of  353  pages,  with  two 
plates.  Cloth,  $2.25. 

HIRST  (BARTON  C.)  AND  PEERSOL.  (GEORGE  A.).  HUMAN 

MONSTROSITIES.  Magnificent  folio,  containing  220  pages  of  text 
and  illustrated  with  123  engravings  and  39  large  photographic  plates 
from  nature.  In  four  parts,  price  each,  $5.  Limited  edition.  For  sale 
by  tubtcription  only. 

HOBLYN  (RICHARD  D.).  A  DICTIONARY  OF  THE  TERMS 
USED  IN  MEDICINE  AND  THE  COLLATERAL  SCIENCES. 

New  (13th)  edition.  In  one  12mo.  volume  of  845  pages.  Cloth, 
$3.00,  net.  Just  ready. 

HODGE  (HUGH  L,.).  ON  DISEASES  PECULIAR  TO  WOMEN, 
INCLUDING  DISPLACEMENTS  OF  THE  UTERUS.  Second  and 
revised  edition.  In  one  8vo.  vol.  of  519  pp.,  with  illus.  Cloth,  $4.50. 

HOFFMANN  (FREDERICK)  AND  POWER  (FREDERICK  B.). 

A  MANUAL  OF  CHEMICAL  ANALYSIS,  as  Applied  to  the 
Examination  of  Medicinal  Chemicals  and  their  Preparations.  Third 
edition,  entirely  rewritten  and  much  enlarged.  In  one  handsome  octavo 
volume  of  621  pages,  with  179  engravings.  Cloth,  $4.25. 

HOLMES  (TIMOTHY).  A  TREATISE  ON  SURGERY.  Its  Prin- 
ciples and  Practice.  A  new  American  from  the  fifth  English  edition. 
Edited  by  T.  PICKERING  PICK,  F.R.C.S.  In  one  handsome  octavo  vol- 
ume of  1008  pages,  with  428  engravings.  Cloth,  $6 ;  leather,  $7. 


A  SYSTEM  OF  SURGERY.  With  notes  and  additions  by  various 

American  authors.  Edited  by  JOHN  H.  PACKARD,  M.  D.  In  three 
very  handsome  8vo.  volumes  containing  3137  double-columned  pages, 
with  979  engravings  and  13  lithographic  plates.  Per  volume,  cloth,  $6  ; 
leather,  $7  ;  half  Russia,  $7.50.  For  tale  by  subscription  only. 

HORNER  (WIL.LJAM  E.).  SPECIAL  ANATOMY  AND  HIS- 
TOLOGY. Eighth  edition,  revised  and  modified.  In  two  large  8vo. 
volumes  of  1007  pages,  containing  320  engravings.  Cloth,  $6. 

HUDSON  (A.).  LECTURES  ON  THE  STUDY  OF  FEVER.    In  one 

octavo  volume  of  308  pages.    Cloth,  $2.50. 

HUTCHISON  (ROBERT)  AND  RAINY  (HARRY).  CLINICAL 
METHODS.  A  GUIDE  TO  THE  PRACTICAL  STUDY  OF 
MEDICINE.  In  one  12mo.  volume  of  562  pages,  with  137  engrav- 
ings and  8  colored  plates.  Cloth,  $3.00. 


A  comprehensive,  clear  and  re- 
ruide  to  clinical 


illustrations    are 


plentiful   and   excellent. — Montreal 
Medical  Journal. 


16     LEA  BBOTHEBS  &  Co.,  PHILADELPHIA  AND  NEW  YORK. 


HYDE  (JAMES  NEVINS).  A  PRACTICAL  TREATISE  ON  DIS- 
EASES OF  THE  SKIN.  New  (5th)  edition,  thoroughly  revised. 
Octavo,  866  pages,  with  111  engravings  and  24  full-page  plates,  8  of 
which  are  colored.  Just  ready.  Cloth,  $4.50,  net;  leather,  $5.50,  net ; 
half  Morocco,  $6.00,  net. 
This  edition  has  been  carefully  re-  j  culcated  throughout  is  sound  as  well 

vised,   and  every  real  advance  has    as  practical. — The  American  Jour- 


been  recognized.  The  work  answers 
the  needs  of  the  general  practitioner, 
the  specialist,  and  the  student. — The 
Ohio  Med.  Jour. 

A  treatise  of  exceptional  merit 
characterized  by  conscientious  care 
and  scientific  accuracy.  —  Buffalo 
Med.  Journal. 

A   complete 


exposition    of    our 


knowledge  of  cutaneous  medicine  as 


it  exists  to-day.    The  teaching  in-  j  corder. 


nal  of  the  Medical  Sciences. 

It  is  the  best  one-volume  work 
that  we  know. —  Virginia  Medical 
Semi-Monthly. 

A  full  and  thoroughly  modern 
text-book  on  dermatology.  —  The 
Pittsburg  Medical  Review. 

The  most  practical  handbook  on 
dermatology  with  which  we  are  ac- 
quainted. —  Chicago  Medical  Re- 


JACKSON  (GEORGE  THOMAS).  THE  READY-REFERENCE 
HANDBOOK  OF  DISEASES  OF  THE  SKIN.  New  (3d)  edition. 
In  one  12mo.  volume  of  637  pages,  with  75  illustrations  and  a  colored 
plate.  Cloth,  $2.50,  net. 

Without  doubt  forms  one  of  the 
best  guides  for  the  beginner  in  der- 
matology that  is  to  be  found  in  the 


As  a  student's  manual,  it  may  be 
considered  beyond  criticism.  The 
book  is  singularly  full. — St.  Louis 
Medical  and  Surgical  Journal. 


English  language. — Medicine. 


JAMIESON  (W.  ALLAN).  DISEASES  OF  THE  SKIN.  Third 
edition.  In  one  octavo  volume  of  656  pages,  with  1  engraving  and  9 
double-page  chromo-lithographic  plates.  Cloth,  $6. 


JEWETT  (CHARLES).  ESSENTIALS  OF  OBSTETRICS.    In  one 

12mo.  volume  of  356  pages,  with  80  engravings  and  3  colored  plates. 
Cloth,  $2.25. 


An  exceedingly  useful  manual  for 
student  and  practitioner.  The  au- 
thor has  succeeded  unusually  well 
in  condensing  the  text  and  in  arrang- 


ing it  in  attractive  and  easily  tangi- 
ble form.  The  book  is  well  illus- 
trated throughout. — Nashville  Jour, 
of  Medicine  and  Surgery. 


THE  PRACTICE  OF  OBSTETRICS.     By   American    Authors. 

One  large  octavo  volume  of  763  pages,  with  441  engravings  in  black 
and    colors,    and    22    full-page  colored   plates.      Cloth,   $5.00,  net; 
leather,  $6.00,  net;  half  Morocco,  $6.50,  net. 
A  clear  and  practical  treatise  upon  I  the  book    abounds.     The  work  is 

obstetrics  by  well-known  teachers  of   sure  to  be  popular    with    medical 

,1  1      •  I  •         1  /•       _    A_  _/»  _J.__   J 11 1.          * _.T 


the  subject.  A  special  feature  of 
this  work  would  seem  to  be  the 
excellent  illustrations  with  which 


students,  as  well  as  being  of  extreme 


value  to    the 
Medical  Age. 


practitioner.  —  The 


JONES  (C.  HANDFIELD).  CLINICAL  OBSERVATIONS  ON 
FUNCTIONAL  NERVOUS  DISORDERS.  Second  American  edi- 
tion. In  one  octavo  volume  of  340  pages.  Cloth,  $3.25. 


LEA  BROTHERS  &  Co.,  PHILADELPHIA  AND  NEW  YORK.     17 

JUL.ER  (HENRY).  A  HANDBOOK  OF  OPHTHALMIC  SCIENCE 
AND  PRACTICE.  Second  edition.  In  one  octavo  volume  of  549 
pages,  with  201  engravings,  17  chromo-lithographic  plates,  test-types  of 
Jaeger  and  Snellen,  and  Holmgren's  Color-Blindness  Test.  Cloth, 
$5.50 ;  leather,  $6.50. 
The  volume  is  particularly  rich  in  |  color  blindness,  etc.  The  sections 

matter  of  practical  value,  such  as  |  devoted  to  treatment  are  singularly 

directions    for    diagnosing,    use    of   full  and  concise. — Medical  Age. 

instruments,  testing  for  glasses,  for  | 

KING  ( A.  F.  A.).  A  MANUAL  OF  OBSTETRICS.  New  (8th)  edition. 
In  one  12mo.  volume  of  612  pages,  with  264  illustrations.  Cloth, 
$2.50,  net.  Just  ready. 


From  first  to  finish  it  is  thoroughly 
practical,  concise  in  expression,  well 
illustrated,  and  includes  a  statement 
of  nearly  every  fact  of  importance 


cyclopedias.  The  well-arranged 
index  renders  the  book  useful  to 
the  practitioner  who  is  in  haste  to 
refresh  his  memory.  —  Virginia 


discussed  in    obstetric    treatises  or  I  Medical  Semi-Monthly. 

KIRK  (EDWARD    C.).      OPERATIVE  DENTISTRY.     Handsome 
octavo  of  700  pages,  with  751  illustrations.    See  American  Text-Book* 
of  Dentistry,  page  2. 
We  have  only  the  highest  praise    tempted.     We  can  heartily  recom- 


mend   it    to    the    profession. — The 
Ohio  Dental  Journal. 


for  this  valuable  work.  It  is  replete 
in  every  particular,  and  surpasses 
anything  of  the  kind  heretofore  at- 

KL.EIN  (E.).  ELEMENTS  OF  HISTOLOGY.  New  (5th)  edition.  In 
one  12mo.  volume  of  506  pages,  with  296  engravings.  Cloth,  $2.00, 
net.  See  Student's  Series  of  Manuals,  page  27. 


It  is  the  most  complete  and  con- 
cise work  of  the  kind  that  has  yet 
emanated  from  the  press. — TheMed- 
ical  Age. 


This  work  deservedly  occupies  a 
first  place  as  a  text-book  on  his- 
tology.— Canadian  Practitioner. 


LANDIS  (HENRY  G.).  THE  MANAGEMENT  OF  LABOR.  In  one 

handsome  12mo.  volume  of  329  pages,  with  28  illus.   Cloth,  $1.75. 

LAURENCE  (J.  Z.)  AND  MOON  (ROBERT  C.).  A  HANDY- 
BOOK  OF  OPHTHALMIC  SURGERY.  Second  edition.  In  one 
octavo  volume  of  227  pages,  with  66  engravings.  Cloth,  $2.75. 

LEA'S  SERIES  OF  POCKET  TEXT-BOOKS,  edited  by  BERN 
B.  GALLATJDET,  M.  D.  Covering  the  entire  field  of  Medicine  in  a 
series  of  16  very  handsome  12mo.  volumes  of  350-450  pages  each, 
profusely  illustrated.  Compendious,  clear,  trustworthy  and  modern. 
The  following  volumes  constitute  the  series. 

COATES'  Bacteriology  and  Hygiene.  BROCKWAY'S  Anatomy.  COLLINS 
and  ROCKWELL'S  Physiology.  MARTIN  and  ROCKWELL'S  Chemistry 
and  Physics.  NICHOLS  and  VALE'S  Histology  and  Pathology. 
SCHLEIF'S  Materia  Medica,  Therapeutics,  Medical  Latin,  etc.  MALS- 
BARY'S  Practice  of  Medicine.  COLLINS'  Diagnosis.  POTTS'  Nervous 
and  Mental  Diseases.  GALLAUDET'S  Surgery.  GRINDON'S  Der- 
matology. WIPPERN  and  BALLENGER'S  Diseases  of  the  Eye,  Ear, 
Throat  and  Nose.  EVANS'  Obstetrics.  CROCKETT'S  Gyuecology. 
TUTTLE'S  Diseases  of  Children. 

For  separate  notices  see  under  various  authors'  names. 


18     LEA  BROTHERS  &  Co.,  PHILADELPHIA  AND  NEW  YORK. 

LEA  (HENRY  C.).  A  HISTORY  OF  AURICULAR  CONFESSION 
AND  INDULGENCES  IN  THE  LATIN  CHURCH.  In  three 
octavo  volumes  of  about  500  pages  each.  Per  volume,  cloth,  $3.00. 

CHAPTERS  FROM  THE  RELIGIOUS  HISTORY  OF  SPAIN ; 

CENSORSHIP  OF  THE  PRESS;  MYSTICS  AND  ILLUMINATI- 
THE  ENDEMONIADAS ;  EL  SANTO  NINO  DE  LA  GUARDIA ; 
BRIANDA  DE  BARDAXI.  12mo.,  522  pages.  Cloth,  $2.50. 


—  FORMULARY  OF  THE   PAPAL  PENITENTIARY.    In  one 
octavo  volume  of  221  pages,  with  frontispiece.    Cloth,  $2.50. 

—  SUPERSTITION  AND  FORCE;  ESSAYS  ON  THE  WAGER 
OF   LAW,  THE   WAGER   OF   BATTLE,  THE  ORDEAL  AND 
TORTURE.      Fourth  edition,  thoroughly   revised.      In   one   hand- 
some royal  12mo.  volume  of  629  pages.     Cloth,  $2.75. 

-  STUDIES  IN  CHURCH  HISTORY.  The  Rise  of  the  Temporal 
Power — Benefit  of  Clergy — Excommunication.  New  edition.  In  one 
handsome  12mo.  volume  of  605  pages.  Cloth,  $2.50. 


AN  HISTORICAL  SKETCH  OF  SACERDOTAL  CELIBACY 

IN  THE  CHRISTIAN  CHURCH.  Second  edition.  In  one  hand- 
some octavo  volume  of  685  pages.  Cloth,  $4.50. 

LOOMS     (ALFRED    L.)    AND    THOMPSON    (W.    OILMAN, 

EDITORS).     A  SYSTEM   OF    PRACTICAL    MEDICINE.      In 

Contributions  by  Various  American  Authors.  In  four  very  hand- 
some octavo  volumes  of  about  900  pages  each,  fully  illustrated  in 
in  black  and  colors.  Complete  work  noiv  ready.  Per  volume,  cloth, 
$5 ;  leather,  $6 ;  half  Morocco,  $7.  For  sale  by  subscription  only. 
Full  prospectus  free  on  application  to  the  Publishers.  See  American 
System  of  Practical  Medicine,  page  2. 

LYMAN  (HENRY  M.).    THE  PRACTICE  OF  MEDICINE.    In  one 

very  handsome  octavo  volume  of  925  pages,  with  170  engravings. 
Cloth,  $4.75 ;  leather,  $5.75. 


Complete,  concise,  fully  abreast  of 
the  times  and  needed  by  all  students 
and  practitioners. —  Univ.  Med.  Mag. 


An  exceedingly  valuable  text-book. 
Practical,  systematic,  and  well  bal- 
anced.— Chicago  Med.  Recorder. 


LYONS  (ROBERT  D.).    A  TREATISE  ON  FEVER.    In  one  octavo 
volume  of  362  pages.     Cloth,  $2.25. 

MACKENZIE  (JOHN  NOLAND).  ON  THE  NOSE  AND  THROAT. 
Handsome  octavo,  about  600  pages,  richly  illustrated.     Preparing. 


LEA  BROTHERS  &  Co.,  PHILADELPHIA  AND  NEW  YORK.     19 


MAISCH  (JOHN  M.).    A    MANUAL    OF    ORGANIC    MATERIA 

MEDICA.  New  (7th)  edition,  thoroughly  revised  by  H.  C.  C.  MAISCH, 
Ph.  G.,  Ph.  D.  In  one  very  handsome  12mo.  volume  of  512  pages,  with 
285  engravings.  Cloth,  $2.50,  net. 


Used  as  text-book  in  every  college 
of  pharmacy  in  the  United  States 
and  recommended  in  medical  col- 
leges.— American  Therapist. 

Noted  on  both  sides  of  the  Atlantic 
and  esteemed  as  much  in  Germany  as 


in  America.  The  work  has  no  equal. 
— Dominion  Med.  Monthly. 

The  best  handbook  upon  phar- 
macognosy  of  any  published  in  this 
country. — Boston  Med.  &  Sur.  Jonr. 


MALSBARY  (GEORGE  E.).  A  POCKET  TEXT-BOOK-  OF 
THEORY  AND  PRACTICE  OF  MEDICINE.  In  one  handsome 
12mo.  volume  of  405  pages,  with  45  illustrations.  Just  ready..  Cloth, 
$1.75,  net;  flexible  red  leather,  $2.25,  net.  Lea's  Series  of  Pocket 
Text-books,  edited  by  BERN  B.  GALLAUDET,  M.  D.  See  page  17. 

Will  readily  commend  itself  to 
students  and  busy  practitioners, 
bringing  forward  as  it  does  the  most 
recent  advances  in  medicine  with 
the  best  of  that  which  is  old.  It 


deals  briefly  and  systematically  with 
each  disease,  as  to  its  history,  aetiol- 
ogy, symptomatology,  diagnosis, 
prognosis  and  treatment. — Medical 
Review  of  Reviews. 


MANUALS.  See  Student's  Quiz  Series,  page  27,  Student's  Series  of 
Manuals,  page  27,  and  Series  of  Clinical  Manuals,  page  25. 

MARSH  (HOWARD).  DISEASES  OF  THE  JOINTS.  In  one  12mo. 
volume  of  468  pages,  with  64  engravings  and  a  colored  plate.  Cloth,  $2. 
See  Series  of  Clinical  Manuals,  page  25. 

MARTIN  (EDWARD).  A  MANUAL  OF  SURGICAL  DIAGNOSIS. 
In  one  12mo.  volume  of  about  400  pp.,  fully  illustrated.  Preparing. 

MARTIN  (WALTON)  AND  ROCKWELL  (WM.  H.).  A  POCKET 
TEXT-BOOK  OF  CHEMISTRY  AND  PHYSICS.  In  one  hand- 
some 12mo.  volume  of  366  pages,  with  137  illustrations.  Just  ready. 
Cloth,  $1.50,  net ;  limp  leather,  $2.00,  net.  Lea's  Series  of  Pocket 
Text-Books,  edited  by  BERN  B.  GALLAUDET,  M.  D.  See  page  17. 


Contains  everything  of  the  sci- 
ences of  chemistry  and  physics 
necessary  for  the  medical  student 
and  practitioner.  The  work  accu- 


rately reflects  both  sciences  in  their 
present  development.  The  arrange- 
ment of  the  matter  is  excellent. — 
The  Medical  and  Surgical  monitor. 


MAY  (O.  H.).    MANUAL  OF  THE  DISEASES  OF  WOMEN.    For 

the  use  of  Students  and  Practitioners.  Second  edition,  revised  by  L. 
S.  RAU,  M.  D.  In  one  12mo.  volume  of  360  pages,  with  31  engrav- 
ings. Cloth,  $1.75. 

MEDICAL  NEWS  POCKET  FORMULARY,  see  page  32. 


20     LEA  BROTHERS  &  Co.,  PHILADELPHIA  AND  NEW  YORK. 


MITCHELL,  (S.  WEIR).  CLINICAL  LESSONS  ON  NERVOUS 
DISEASES.  In  one  12mo.  volume  of  299  pages,  with  19  engravings 
and  2  colored  plates.  Cloth,  $2.50. 


The  book  treats  of  hysteria,  recur- 
rent melancholia,  disorders  of  sleep, 
choreic  movements,  false  sensations 
of  cold,  ataxia,  hemiplegic  pain, 
treatment  of  sciatica,  erythromelal- 


contractions,  rotary  movements  in 
the  feeble  minded,  etc.  Few  can 
speak  with  more  authority  than  the 
author. —  The  Journal  of  the  Ameri- 
can Medical  Association. 


gia,  reflex  ocularneurosis,  hysteric 

MITCHELL,  (JOHN  K.).  REMOTE  CONSEQUENCES  OF  IN- 
JURIES OF  NERVES  AND  THEIR  TREATMENT.  In  one 
handsome  12mo.  volume  of  239  pages, with  12  illustrations.  Cloth,  $1.75. 


MORRIS  (MALCOLM).  DISEASES  OF  THE  SKIN.  New  (2d) 
edition.  In  one  12mo.  volume  of  601  pages,  with  10  chromo-litho- 
graphic  plates  and  26  engravings.  Cloth,  $3.25,  net. 


The  work  is  essentially  clinical 
and  practical  in  its  scope  and  is 
characterized  throughout  by  clear- 
ness and  simplicity  of  style  and 


strong  common  sense.  It  is  alike 
suitable  for  the  student,  physician 
and  specialist.  —  Buffalo  Medical 
Journal. 


MULLER  (J.).    PRINCIPLES  OF  PHYSICS   AND  METEOROL- 
OGY.   In  one  large  8vo.  vol.  of  623  pages,  with  538  cuts.  Cloth,  $4.50. 


MUSSER  (JOHN  H.).  A  PRACTICAL  TREATISE  ON  MEDICAL 
DIAGNOSIS,  for  Students  and  Physicians.  New  (3d)  edition,  thor- 
oughly revised.  In  one  octavo  volume  of  1082  pages,  with  253  en- 
gravings and  48  full-page  colored  plates.  Cloth,  $6.00,  net ;  leather, 
$7.00,  net;  half  Morocco,  $7.50,  net. 

We  have  no  work  of  equal  value  It  so  thoroughly  meets  the  precise 
in  English.  —  University  Medical  \  demands  incident  to  modern  research 
Magazine.  '<  that  it  has  been  adopted  as  the  lead- 

From  its  pages  may  be  made  the  j  in,g  text-book  by  the  medical  colleges 
diagnosis  of  every  malady  that  I  of  thls  country.—  North  American 
afflicts  the  human  body,  including  |  Practitioner. 

those  which  in  general    are  dealt  I      The  best  of  its  kind,  invaluable  to 
with  only  by  the  specialist. — North-    the  student,  general  practitioner  and 


western  Lancet. 


teacher. — Montreal  Medical  Journa  I. 


NATIONAL  DISPENSATORY.  See  Stille,  Maisch  &  Caspari,  p.  27. 

NATIONAL  FORMULARY.   See  Stille,  Mauch  &  Caspari' s  National 
Dispensatory,  page  27. 


NATIONAL  MEDICAL  DICTIONARY.    See  killings,  page  4, 


LEA  BROTHERS  &  Co.,  PHILADELPHIA  AND  NEW  YORK.     21 


NETTLiESHIP  (E.).  DISEASES  OF  THE  EYE.  New  (6th)  American 
from  sixth  English  edition,  thoroughly  revised.  In  one  12mo.  volume 
of  562  pages,  with  192  engravings,  and  5  colored  plates,  test-types, 
formulae  and  color-blindness  test.  Cloth,  $2.25,  net.  Just  ready. 


By  far  the  best  student's  text-book 
on  the  subject  of  ophthalmology. — 
The  Clinical  Review. 

This  work  for  compactness,  practi- 
cality and  clearness  has  no  superior 
in  the  English  language. — Journal 
of  Medicine  and  Science. 


The  present  edition  is  the  result 
of  revision  both  in  England  and 
America,  and  therefore  contains  the 
latest  and  best  ophthalmological 
ideas  of  both  continents. — The  Phy- 
sician and  Surgeon. 


NICHOLS  (JOHN  B.)  AND  VALE  (F.  P.).  A  POCKET  TEXT- 
BOOK OF  HISTOLOGY  AND  PATHOLOGY.  In  one  handsome 
12mo.  volume  of  452  pages,  with  213  illustrations.  Just  ready.  Cloth, 
$1.75,  net:  flexible  red  leather,  $2.25,  net. 

Lea's  Series  of  Pocket  Text-books,  edited  by  BERN  B.  GALLATJDET, 
M.  D.     See  page  17. 


So  systematically  arranged  that  it 
is,  in  the  highest  degree,  interesting. 
Thoroughly  up  to  date.  The  book 


can  safely  and  conscientiously  rec- 
ommend it  to  both  students  and 
practitioners. — The  St.  Louis  Medi- 


is  an  exceptionally  good  one.     We  j  cat  and  Surgical  Journal. 

NORRIS  (WM.  F.)  AND  OLIVER  (CHAS.  A.).  TEXT-BOOK  OF 

OPHTHALMOLOGY.    In  one  octavo  volume  of  641  pages,  with  357 
engravings  and  5  colored  plates.     Cloth,  $5 ;  leather,  $6. 


has  ever  been  offered  to  the  Amer- 
ican medical  public. — Annals  of 
Ophthalmology  and  Otology. 


It  is  practical  in  its  teachings. 
We  unreservedly  endorse  it  as  the 
best,  the  safest  and  the  most  compre- 
hensive volume  upon  the  subject  that 

OWEN    (EDMUND).     SURGICAL   DISEASES    OF    CHILDREN. 

In  one  12mo.  volume  of  525  pages,  with  85  engravings  and  4  colored 
plates.  Cloth,  $2.  See  Series  of  Clinical  Manuals,  page  25. 

PARK  (ROSWEL.L,).  A  TREATISE  ON  SURGERY  BY  AMERI- 
CAN AUTHORS.  New  and  condensed  edition.  In  one  royal  octavo 
volume  of  1261  pages,  with  625  engravings  and  37  full-page  plates. 
Cloth,  $6.00,  net;  leather,  $7.00,  net. 

^^"•This  work  is  also  published  in  a  larger  edition,  comprising  two 
volumes.  Volume  I.,  General  Surgery,  799  pages,  with  356  engravings 
and  21  full-page  plates,  in  colors  and  monochrome.  Volume  II., 
Special  Surgery,  800  pages,  with  430  engravings  and  17  full-page 
plates,  in  colors  and  monochrome.  Per  set,  cloth,  $9.00 ;  leather, 
$11.00,  net;  half  Morocco,  $12.00,  net. 


The  work  is  fresh,  clear  and  practi- 
cal, covering  the  ground  thoroughly 
yet  briefly,  and  well  arranged  for 
rapid  reference,  so  that  it  will  be  of 
special  value  to  the  student  and  busy 
practitioner.  The  pathology  is 
broad,  clear  and  scientific,  while  the 
suggestions  upon  treatment  are 


clear-cut,  thoroughly  modern  and 
admirably  resourceful. — Johns  Hop- 
king  Hospital  Bulletin. 

The  latest  and  best  work  written 
upon  the  science  and  art  of  surgery. 
Columbus  Medical  Journal. 

It  is  thoroughly  practical  and  yet 
thoroughly  scientific. — Med.  News. 


22     LEA  BBOTHBBS  &  Co.,  PHILADELPHIA  AND  NEW  YORK. 

PARK  (WILLIAM  H.).     BACTERIOLOGY  IN  MEDICINE  AND 

SURGERY.     12mo.,  688  pages,  with  87  illustrations  in  black  and 
colors,  and  2  plates.     Cloth,  $3.00  net. 


This  book  fills  a  very  distinct 
gap.  None  of  the  text-books  in  our 
language  take  up  the  subject  of  bac- 
teriology so  thoroughly  and  so 
soundly  as  does  this  from  the  point 


of  view  of  the  hygienist  and  public 
health  officer.  The  work  is  correct 
and  very  well  up  to  date. — The  Mon- 
treal Medical  Journal. 


PARRY  (JOHN  S.).  EXTRA-UTERINE  PREGNANCY,  ITS 
CLINICAL  HISTORY,  DIAGNOSIS,  PROGNOSIS  AND  TREAT- 
MENT. In  one  octavo  volume  of  272  pages.  Cloth,  $2.50. 

PARVIN  (THEOPHIL.US).    THE  SCIENCE  AND  ART  OF  OB- 

STETRICS.  Third  edition.  In  one  handsome  octavo  volume  of 
677  pages,  with  267  engravings  and  2  colored  plates.  Cloth,  $4.25 ; 
leather,  $5.25. 


Parvin's  work    is  practieal,  con- 
cise and  comprehensive.     We  com- 


English   language. — Medical    Fort- 
nightly. 


mend  it  as  first  of  its  class  in  the 

PEPPER'S  SYSTEM  OF  MEDICINE.    See  page  3. 

PEPPER  (A.  J.).  FORENSIC  MEDICINE.  In  press.  See  Student's 
Series  of  Manuals,  page  27. 

SURGICAL  PATHOLOGY.    In  one  12mo.  volume  of  511  pages, 

with  81  engravings.   Cloth,  $2.   See  Student's  Series  of  Manuals,  p.  27. 

PICK  (T.  PICKERING).  FRACTURES  AND  DISLOCATIONS. 
In  one  12mo.  volume  of  530  pages,  with  93  engravings.  Cloth,  $2. 
See  Series  of  Clinical  Manuals,  page  25. 

PLAYFAIR  (W.  S.).  A  TREATISE  ON  THE  SCIENCE  AND 
PRACTICE  OF  MIDWIFERY.  Seventh  American  from  the  ninth 
English  edition.  In  one  octavo  volume  of  700  pages,  with  207 
engravings  and  7  plates.  Cloth,  $3.75  net ;  leather,  $4.75,  net. 

An  epitome  of  the  science  and  a  safe  guide  to  both  student  and 
practice  of  midwifery,  which  em-  j  obstetrician.  It  holds  a  place  among 
bodies  all  recent  advances.  —  The  \  the  ablest  English-speaking  authori- 


Medical  Fortnightly. 

This  work  must  occupy  a  fore- 
most place  in  obstetric  medicine  as 


ties  on  the  obstetric    art. — Buffalo 
Medical  and  Surgical  Journal. 


—  THE  SYSTEMATIC  TREATMENT  OF  NERVE  PROSTRA- 
TION AND  HYSTERIA.  In  one  12mo.  volume  of  97  pages 
Cloth,  $1. 


LEA  BEOTHKES  &  Co.,  PHILADELPHIA  AND  NEW  YOEK.     23 
POCKET  FORMULARY,  see  page  32. 
POCKET  TEXT-BOOKS,  see  page  18. 

POLITZER  (ADAM).  A  TEXT-BOOK  OF  THE  DISEASES  OF  THE 
EAR  AND  ADJACENT  ORGANS.  Second  American  from  the 
third  German  edition.  Translated  by  OSCAR  DODD,  M.  D.,  and 
edited  by  SIE  WILLIAM  DALBY,  F.  R.  C.  S.  In  one  octavo  volume  of 
748  pages,  with  330  original  engravings. 

POTTS  (CHARLES  S.).  A  POCKET  TEXT-BOOK  OF  NERVOUS 
AND  MENTAL  DISEASES.  In  one  handsome  12mo.  volume  of 
445  pages,  with  88  engravings.  Just  ready.  Cloth,  $1.75,  net ;  limp 
leather,  $2.25,  net.  Lea's  Series  of  Pocket  Text-books,  edited  by 
BEEN  B.  GALLAUDET,  M.  D.  See  page  17. 

Dr.  Potts  has  succeeded  in  de-  of  the  numerous  discoveries  in  every 
picting  the  main  facts  in  a  manner  branch  of  neurology  is  clearly  pre- 
that  will  be  appreciated  by  students  j  sented.  The  book  is  a  reliable  guide, 
and  general  practitioners.  The  gist  |  — The  Medical  Bulletin. 

PROGRESSIVE  MEDICINE,  see  page  32. 

PURDY  (CHARLES  W.).  BRIGHT'S  DISEASE  AND  ALLIED 
AFFECTIONS  OF  THE  KIDNEY.  In  one  octavo  volume  of  288 
pages,  with  18  engravings.  Cloth,  $2. 

PYE-SMITH  (PHDLIP  H.).  DISEASES  OF  THE  SKIN.  In  one 
12mo.  vol.  of  407  pp.,  with  28  illus.,  18  of  which  are  colored.  Cloth,  $2. 

QUIZ  SERIES.     See  Student's  Quiz  Series,  page  27. 

RALFE    (CHARLES  H.).      CLINICAL     CHEMISTRY.     In    one 

12mo.  volume  of  314  pages,  with  16  engravings.    Cloth,  $1.50.    See 
Student's  Series  of  Manuals,  page  27. 

RAMSBOTHAM  (FRANCIS  H.).  THE  PRINCIPLES  AND  PRAC- 
TICE OF  OBSTETRIC  MEDICINE  AND  SURGERY.  In  one 
imperial  octavo  volume  of  640  pages,  with  64  plates  and  numerous 
engravings  in  the  text.  Strongly  bound  in  leather,  $7. 

REMSEN  (IRA).  THE  PRINCIPLES  OF  THEORETICAL  CHEM- 
ISTRY. New  (5th)  edition,  thoroughly  revised.  In  one  12mo.  vol- 
ume of  326  pages.  Cloth,  $2. 


A  clear  and  concise  explanation 
of  a  difficult  subject.  We  cordially 
recommend  it. —  The  London  Lancet. 

The  book  is  equally  adapted  to  the 


student  of  chemistry  or  the  practi- 
tioner who  desires  to  broaden  his 
theoretical  knowledge  of  chemistry. 
— New  Orleans  Med.  and  Surg.  Jour. 


24     LEA  BROTHERS  &  Co.,  PHILADELPHIA  AND  NEW  YORK. 

RICHARDSON  (BENJAMIN  WARD).  PREVENTIVE  MEDI- 
CINE. In  one  octavo  volume  of  729  pages.  Cloth,  $4 ;  leather,  $5. 

ROBERTS  (JOHN  B.).  THE  PRINCIPLES  AND  PRACTICE  OF 
MODERN  SURGERY.  New  (2d)  edition.  In  one  octavo  volume  of 
838  pages  with  473  engravings  and  8  plates.  Just  ready.  Cloth,  $4  25, 
net ;  leather,  $5.25,  net. 

satisfactory  or  valuable  single  vol- 
ume work  on  this  subject. — Pacific 


A  clear,  concise,  comprehensive 
and  practical  presentation  of  the 
most  modern  surgery.  The  student 
or  practitioner  will  not  find  a  more 


Medical  Journal. 


ROBERTS  (SIR  WILLIAM).  A  PRACTICAL  TREATISE  ON 
URINARY  AND  RENAL  DISEASES,  INCLUDING  URINARY 
DEPOSITS.  Fourth  American  from  the  fourth  London  edition.  In 
one  very  handsome  8vo.  vol.  of  609  pp.,  with  81  illus.  Cloth,  $3.50. 

ROSS  (JAMES).  A  HANDBOOK  OF  THE  DISEASES  OF  THE 
NERVOUS  SYSTEM.  In  one  handsome  octavo  volume  of  726  pagee, 
with  184  engravings.  Cloth,  $4.50 ;  leather,  $5.50. 

SCHAFER  (EDWARD  A.).  THE  ESSENTIALS  OF  HISTOL- 
OGY, DESCRIPTIVE  AND  PRACTICAL.  For  the  use  of  Students. 
New  (5th)  edition.  In  one  handsome  octavo  volume  of  359  pages, 
with  392  illustrations.  Cloth,  $3.00,  net. 


Nowhere  else  will  the  same  very 
moderate  outlay  secure  as  thoroughly 
useful  and  interesting  an  atlas  of 
structural  anatomy. — The  American 
Journal  of  the  Medical  Sciences. 


The  most  satisfactory  elementary 
text-book  of  histology  in  the  Eng- 
lish language. — The  Boston  Med.  and 
Sur.  Jour. 


A  COURSE  OF  PRACTICAL  "HISTOLOGY.    New  (2d)  edition. 

In  one  12mo.  volume  of  307  pages,  with  59  engravings.   Cloth,  $2.25. 

SCHLEIF  (WILLIAM).  MATERIA  MEDICA,  THERAPEUTICS, 
PRESCRIPTION  WRITING,  MEDICAL  LATIN,  ETC.  12mo., 
352  pages.  Cloth,  $1.50,  net;  flexible  red  leather,  $2.00,  net.  Just, 
ready.  Lea's  Series  of  Pocket  Text-books.  Edited  by  BERN  B. 
GALLAUDET,  M.  D.  See  page  17. 
We  commend  the  book  for  it  con-  college  courses  on  Matcria  Medica 


tains  in  a  concise,  definite,  and  as- 
similable form  the  essential  knowl- 
edge required  in  the  most  complete 


and    Therapeutics. — The    National, 
Medical  Review. 


LEA  BROTHERS  &  Co.,  PHILADELPHIA  AND  NEW  YOBK.     25 

SCHMITZ  AND  ZUMPT'S  CLASSICAL  SERIES.  Advanced 
Latin  Exercises.  Cloth,  60  cts.  Schmidt's  Elementary  Latin  Exer- 
cises. Cloth,  50  cents.  Sallust.  Cloth,  60  cents.  Nepos.  Cloth,  60 
cents.  Virgil.  Cloth,  85  cents.  Curtius.  Cloth,  80  cents. 

SCHOFIELD  (ALFRED  T.).  ELEMENTARY  PHYSIOLOGY 
FOR  STUDENTS.  In  one  12mo.  volume  of  380  pages,  with  227 
engravings  and  2  colored  plates.  Cloth,  $2. 

SENN  (NICHOLAS).  SURGICAL  BACTERIOLOGY.  Second  edi- 
tion. In  one  octavo  volume  of  268  pages,  with  13  plates,  10  of  which 
are  colored,  and  9  engravings.  Cloth,  $2. 

SERIES  OF  CLINICAL  MANUALS.  A  Series  of  Authoritative 
Monographs  on  Important  Clinical  Subjects,  in  12mo.  volumes  ot  about 
550  pages,  well  illustrated.  The  following  volumes  are  now  ready : 
YEO  on  Food  in  Health  and  Disease,  new  (2d)  edition,  $2.50;  CARTER 
and  FROST'S  Ophthalmic  Surgery,  $2.25 ;  MARSH  on  Diseases  of  the 
Joints,  $2 ;  OWEN  on  Surgical  Diseases  of  Children,  $2 ;  PICK  on 
Fractures  and  Dislocations,  $2  ;  SAVAGE  on  Insanity  and  Allied  Neu- 
roses, $2. 
For  separate  notices,  see  under  various  authors'  names. 

SERIES  OF  STUDENT'S  MANUALS.    See  page  27. 

SIMON  (CHARLES  E.).  CLINICAL  DIAGNOSIS,  BY  MICRO- 
SCOPICAL AND  CHEMICAL  METHODS.  New  (3d)  edition.  In 
one  veiy  handsome  octavo  volume  of  563  pages,  with  138  engravings 
and  18  full-page  colored  plates.  Cloth,  $3.50,  net.  Just  ready. 


This  book  thoroughly  deserves  its 
success.  It  is  a  very  complete,  authen- 
tic and  useful  manual  of  the  micro- 
scopical and  chemical  methods 
which  are  employed  in  diagnosis. 
Very  excellent  colored  plates  illus- 
trate this  work. — New  York  Medical 
Journal. 


In  all  respects  entirely  up  to  date. 
— Medical  Record. 

The  chapter  on  examination  of 
the  urine  is  the  most  complete  and 
advanced  that  we  know  of  in  the 
English  language. — Canadian  Prac- 
titioner. 


SIMON  (W.).  MANUAL  OF  CHEMISTRY.  A  Guide  to  Lectures 
and  Laboratory  Work  for  Beginners  in  Chemistry.  A  Text-book 
specially  adapted  for  Students  of  Pharmacy  and  Medicine.  New  (6th) 
edition.  In  one  8vo.  volume  of  536  pages,  with  46  engravings  and  8 
plates  showing  colors  of  64  tests.  Cloth,  $3.00,  net. 


It  is  difficult  to  see  how  a  better 
book  could  be  constructed.  No  man 
who  devotes  himself  to  the  practice 
of  medicine  need  know  more  about 
chemistry  than  is  contained  between  !  Med.  Monthly. 


the  covers  of  this  book. — The  North- 
western Lancet. 

Its  statements  are  all  clear  and  its 
teachings    are  practical. —  Virginia 


26     LEA  BROTHERS  &  Co.,  PHILADELPHIA  AND  NEW  YORK. 

SLADE  (D.  D.).  DIPHTHERIA;  ITS  NATURE  AND  TREAT- 
MENT. Second  edition.  In  one  royal  12mo.  vol.,  158  pp.  Cloth,  $1.25. 

SMITH  (EDWARD).  CONSUMPTION ;  ITS  EARLY  AND  REME- 
DIABLE STAGES.  In  one  8vo.  volume  of  253  pp.  Cloth,  $2.25. 

SMITH  (J.  LEWIS).  A  TREATISE  ON  THE  DISEASES  OF  IN- 
FANCY AND  CHILDHOOD.  Eighth  edition,  thoroughly  revised 
and  rewritten  and  much  enlarged.  In  one  large  8vo.  volume  of  983 

«,  with  273  engravings  and  4  full-page  plates.    Cloth,  $4.50; 

icr,  $5.50. 


A  safe  guide  for  students  and  phy- 
sicians.— The  Am.  Jour,  of  Obstetrics. 

For  years  the  leading  text-book  on 
children's  diseases  in  America. — 
Chicago  Medical  Recorder. 


The  most  complete  and  satisfac- 
tory text-book  with  which  we  are 
acquainted. — American  Gynecologi- 
cal and  Obstetrical  Journal. 


SMITH  (STEPHEN).  OPERATIVE  SURGERY.  Second  and  thor- 
oughly revised  edition.  In  one  octavo  volume  of  892  pages,  with 
1005  engravings.  Cloth,  $4  ;  leather,  $5. 


One  of  the  most  satisfactory  works 
on  modern  operative  surgery  yet 
published.  The  book  is  a  compen- 


dium for  the  modern  surgeon. — Bos- 
ton Medical  and  Surgical  Journal. 


SOLLY  (S.  EDWIN).  A  HANDBOOK  OF  MEDICAL  CLIMA- 
TOLOGY. In  one  handsome  octavo  volume  of  462  pages,  with  en- 
gravings and  11  full-page  plates,  5  of  which  are  in  colors.  Cloth,  $4.00. 


Every  practitioner  of  medicine 
should  possess  himself  of  a  copy  and 
study  it,  and  we  are  sure  he  will 


A  clear  and  lucid  summary  of 
what  is  known  of  climate  in  relation 
to  its  influence  upon  human  beings. 


never  regret  it. — St.  Louis  Medical  I  — The  Therapeutic  Gazette, 
and  Surgical  Journal. 

STILLE  (ALFRED).  CHOLERA;  ITS  ORIGIN,  HISTORY,  CAUS- 
ATION, SYMPTOMS,  LESIONS,  PREVENTION  AND  TREAT- 
MENT. In  one  12mo.  volume  of  163  pages,  with  a  chart  showing 
routes  of  previous  epidemics.  Cloth,  $1.25. 

THERAPEUTICS   AND    MATERIA    MEDICA.      Fourth   and 

revised  edition.      In  two  octavo  volumes,  containing    1936    pages. 
Cloth,  $10;  leather,  $12. 


STILLE  (ALFRED),  MAISCH  (JOHN  M.)  AND  CASPARI 
(CHAS.  JR.).  THE  NATIONAL  DISPENSATORY:  Containing 
the  Natural  History,  Chemistry,  Pharmacy,  Actions  and  Uses  of 
Medicines  including  those  recognized  in  the  latest  Pharmacopeias  of 
the  United  States,  Great  Britain  and  Germany,  with  numerous  refer- 
ences to  the  French  Codex.  Fifth  edition,  revised  and  enlarged, 
including  the  new  U.  S.  Pharmacopoeia,  Seventh  Decennial  Revision. 
With  Supplement  containing  the  new  edition  of  the  National  Formu- 
lary. In  one  magnificent  imperial  octavo  volume  of  about  2025  pages, 
with  320  engravings.  Cloth,  $7.25;  leather,  $8.  With  ready  reference 
Thumb-letter  Index.  Cloth,  $7.75  ;  leather,  $8.50. 


LEA  BROTHERS  &  Co.,  PHILADELPHIA  AND  NEW  YORK.     27 

STIMSON  (LEWIS  A.).    A  MANUAL  OF  OPERATIVE  SURGERY. 

New  (4th)  edition.   In  one  royal  12mo.  volume  of  581  pages,  with  293 
engravings.     Cloth,  $3.00,  net.    Just  ready. 


A  useful  and  practical  guide  for 
all  students  and  practitioners. — Am. 
Journal  of  the  Medical  Sciences. 


The  book  is  worth  the  price  for  the 
illustrations  alone. — Ohio  Medical 
Journal. 


STIMSON  (LEWIS  A.).     A  TREATISE  ON  FRACTURES    AND 
DISLOCATIONS.  In  one  handsome  octavo  volume    of  831  pages, 
with  326  engravings  and   20  plates.         Cloth,    $5.00,  net;  leather, 
$6.00,  net;  half  Morocco,  $6.50,  net. 
Preeminently   the    authoritative    pensable  to  the  student  and  the  prac- 


text-book  upon  the  subject.  The 
vast  experience  of  the  author  gives 
to  his  conclusions  an  unimpeachable 
value.  The  work  is  profusely  il- 
lustrated. It  will  be  found  indis- 


titioner  alike. — The  Medical  Age. 

The  work  is  the  best  one  in  Eng- 
lish to-day. — St.  Louis  Medical  and 
Surgical  Journal. 


STUDENT'S  QUIZ  SERIES.  Thirteen  volumes,  convenient,  author- 
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of  the  Eye,  Ear,  Throat  and  Nose;  11.  Obstetrics;  12.  Gynecology  ; 
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CLARKE  and  LOCKWOOD'S  Dissector's  Manual,  $1.50. 
For  separate  notices,  see  under  various  author's  names. 

STURGES  (OCTAV1US).  AN  INTRODUCTION  TO  THE  STUDY 
OF  CLINICAL  MEDICINE.  In  one  12mo.  volume.  Cloth,  $1.25. 

SUTTON  (JOHN  BLAND).  SURGICAL  DISEASES  OF  THE 
OVARIES  AND  FALLOPIAN  TUBES.  Including  Abdominal 
Pregnancy.  In  one  12mo.  volume  of  513  pages,  with  119  engravings 
and  5  colored  plates.  Cloth,  $3. 

TAIT  (LAWSON).  DISEASES  OF  WOMEN  AND  ABDOMINAL 
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28     LEA  BROTHERS  &  Co.,  PHILADELPHIA  AND  NEW  YOEK. 

TANNER  (THOMAS  HAWKES)  ON  THE  SIGNS  AND  DIS- 
EASES OF  PREGNANCY.  From  the  second  English  edition.  In 
one  octavo  volume  of  490  pages,  with  4  colored  plates  and  16  engrav- 
ings. Cloth,  $4.25. 

TAYLOR  (ALFRED  S.).     MEDICAL   JURISPRUDENCE.     New 

American  from  the  twelfth  English  edition,  specially  revised  by  CLARK 
BELL,  ESQ.,  of  the  N.  Y.  Bar.    In  one  8vo.  vol.  of  831  pages,  with  54 
engrs.  and  8  full-page  plates.     Cloth,  $4.50;  leather,  $5.50. 
To  the  student,  as  to  the  physician,  f  be  found  to  be  thorough,  authorita- 

we  would  say,  get  Taylor  first,  and  I  tive    and     modern. — Albany    Law 

then  add  as  means  and  inclination    Journal. 


enable  you. — American  Practitioner 
and  News. 

It  is  the  authority  accepted  as 
final  by  the  courts  of  all  English- 
speaking  countries.  The  work  will 


Probably  the  best  work  on  the 
subject  written  in  the  English  lan- 
guage. The  work  has  been  thor- 
oughly revised  and  is  up  to  date. — 
Pacific  Medical  Journal. 


ON  POISONS  IN  RELATION  TO  MEDICINE  AND  MEDI- 
CAL JURISPRUDENCE.  Third  American  from  the  third  London 
edition.  In  one  octavo  volume  of  788  pages,  with  104  illustrations. 
Cloth,  $5.50;  leather,  $6.50. 

TAYLOR  (ROBERT  W.).     THE    PATHOLOGY  AND   TREAT- 
MENT OF  VENEREAL  DISEASES.  New  (2d)  edition.   In  one  very 
handsome  octavo  volume  of  about  800  pages,  with  about  250  engrav- 
ings and  many  colored  plates.    Shortly. 
Notices  of  previous  edition  are  appended. 

By  long  odds  the  best  work  on  |      It  is  a  veritable  storehouse  of  our 

venereal  diseases. — Louisville  Medi-    knowledge  of  the  venereal  diseases. 

cal  Monthly.  It  is  commended  as   a  conservative, 

The  clearest,  most  unbiased  and    practical,    full    exposition     of  the 


greatest    value.  — Chicago    Clinical 
Review. 


ably  presented  treatise  as  yet  pub 
lished  on  this  vast  subject. — The 
Medical  News. 

TAYLOR  (ROBERT  W.).    A  PRACTICAL  TREATISE  ON  SEX- 
UAL   DISORDERS  IN  THE  MALE  AND   FEMALE.     New  (2d) 
edition.    In  one  8vo.  volume  of  434  pages,  with  91  engravings  and 
13  colored  plates.     Cloth,  $3.00,  net.    Just  ready. 
The  author  has  presented  to  the  j  followed,  will  be  of  unlimited  value 

profession  the  ablest  and  most  scien-  j  to    both   physician    and   patient. — 

tific  work  as  yet  published  on  sexual    Medical  News. 

disorders,  and  one  which,  if  carefully 

A  CLINICAL  ATLAS  OF  VENEREAL  AND  SKIN  DISEASES. 

Including  Diagnosis,  Prognosis  and  Treatment.  In  eight  large  folio 
parts,  measuring  14  x  18  inches,  and  comprising  213  beautiful  figures 
on  58  full-page  chromo-lithographic  plates,  85  fine  engravings  and  425 
pages  of  text.  Complete  work  now  ready.  Price  per  part,  sewed  in 
heavy  embossed  paper,  $2.50.  Bound  in  one  volume,  half  Russia, 
$27  ;  half  Turkey  Morocco,  $28.  For  sale  by  subscription  only.  Address 
the  publishers.  Specimen  plates  by  mail  on  receipt  often  cents. 

TAYLOR  (SEYMOUR).  INDEX  OF  MEDICINE.  A  Manual  for 
the  use  of  Senior  Students  and  others.  In  one  large  12mo.  volume  of 
802  pages.  Cloth,  $3.76. 


LEA  BROTHERS  &  Co.,  PHILADELPHIA  AND  NEW  YORK.    29 


THOMAS  (T.  GAILLiARD)  AND  MTJNDE  (PAULi  P.).  A  PRAC- 
TICAL TREATISE  ON  THE  DISEASES  OF  WOMEN.  Sixth 
edition,  thoroughly  revised  by  PAUL  F.  MTJNDE,  M.  D.  In  one 
large  and  handsome  octavo  volume  of  824  pages,  with  347  engravings. 
Cloth,  $5 ;  leather,  $6. 
The  best  practical  treatise  on  the 

subject  in    the    English    language. 

It  will  be  of  especial  value  to  the 


general  practitioner  as  well  as  to  the 
specialist.  The  illustrations  are  very 
satisfactory.  Many  of  them  are  new 
and  are  particularly  clear  and  attrac- 
tive.— JBoston  Med .  and  Sur.  Jour. 


This  work,  which  has  already  gone 
through  five  large  editions,  and  has 
been  translated  into  French,  Ger- 


UC^JJ.        UCfeUBACbWU       AAil/Vf       J_   A  t/AJ.v>JJ.j       VJlv/J. 

man,  Spanish  and  Italian,  is  the 
most  practical  and  at  the  same  time 
the  most  complete  treatise  upon  the 
subject. — The  Archives  of  Gynecol- 
ogy,  Obstetrics  and  Pediatrics. 


THOMPSON  (W.  OILMAN).  A  TEXT-BOOK  OF  PRACTICAL 
MEDICINE.  For  Student's  and  Practitioners.  In  one  handsome 
octavo  volume  of  1012  pages,  with  79  engravings.  Just  ready. 
Cloth,  $5.00,  net;  leather,  $6.00,  net;  half  Morocco,  $6.50,  net. 

THOMPSON  (SLR  HENRY).  CLINICAL  LECTURES  ON  DIS- 
EASES OF  THE  URINARY  ORGANS.  Second  and  revised  edi- 
tion. In  one  octavo  vol.  of  203  pp.,  with  25  engravings.  Cloth,  $2.25. 


THE    PATHOLOGY   AND   TREATMENT   OF   STRICTURE 

OF  THE  URETHRA  AND  URINARY  FISTULA.  From  the 
third  English  edition.  In  one  octavo  volume  of  359  pages,  with  47 
engravings  and  3  lithographic  plates.  Cloth,  $3.50. 

THOMSON  (JOHN).    DISEASES  OF  CHILDREN.     In  one  crown 

octavo  volume  of  350  pages,  with  52  illus.  Cloth,  $1.75,  net. 
In  this  admirable  work  the  sub-  encroach  upon  any  existing  work, 
ject  is  approached  from  a  purely  It  contains  many  things  not  to  be 
clinical  stand-point.  It  differs  from  found  in  the  text-books,  and  is  prac- 
anything  that  has  yet  appeared  upon  tical  in  the  extreme. — Archives  of 
diseases  of  children,  and  does  not '  Pediatrics. 

TIRARD  (NESTOR).  MEDICAL  TREATMENT  OF  DISEASES 
AND  SYMPTOMS.  Handsome  octavo  volume  of  627  pages.  Just 
ready.  Cloth,  $4.00,  net. 

This  work  will  rapidly  come  into  !  this  is  a  work  destined  to  become 

popular,  and  we  take  great  pleasure 
in   commending  it  in  the    highest 


terms. — Nashville  Journal  of  Medi- 
cine and  Surgery. 


favor  with  students  and  practition- 
ers. It  deals  comprehensively  with 
therapeutical  medications  and  pre- 
sents a  great  number  of  well-selected 
formulas  of  every  day  use.  Certainly 

TODD  (ROBERT  BENTL.EY).     CLINICAL  LECTURES  ON  CER- 
TAIN ACUTE  DISEASES.    In  one  8vo.  vol.  of  320  pp.,  cloth,  $2.50. 

TREVES    (FREDERICK).      OPERATIVE    SURGERY.      In    two 

8vo.  vols.  containing  1550  pp.,  with  422  illus.     Cloth,  $9 ;  leath.,  $11. 

A  SYSTEM  OF  SURGERY.    In  Contributions  by  Twenty-five 

English  Surgeons.  In  two  large  octavo  volumes.  Vol.  I.,  1178  pages, 
with  463  engravings  and  2  colored  plates.  Vol.  II.,  1120  pages,  with 
487  engravings  and  2  colored  plates.  Complete  work,  cloth,  $16.00. 


30     LEA  BROTHERS  &  Co.,  PHILADELPHIA  AND  NEW  YORK. 


TREVES  (FREDERICK).    SURGICAL  APPLIED  ANATOMY.  In 

one  12mo.  volume  of  540  pages,  with  61  engravings.     Cloth,  $2.     See 
Student's  Series  of  Manuals,  page  27. 

TUTTL.E  (GEORGE  M.).  A  POCKET  TEXT-BOOK  OF  DISEASES 
OF  CHILDREN.  In  one  handsome  12mo.  volume  of  374  pages, 
with 5  plates.  Just  ready.  Cloth,  $1.50,  net;  flexible  red  leather, 
$2.00,  net.  Lea's  Seri.es  of  Pocket  Text-books,  edited  by  BERN  B. 
GALLATJDET,  M.  D.  See  p.  17. 


It  is  a  good  work — the  author  hav- 
ing condensed  most  of  the  leading 
points  in  connection  with  diseases 


of  infancy  and  childhood  into  short 
and  readable  chapters. —  Virginia 
Medical  Semi-Monthly. 


VAT7GHAN    (VICTOR    C.)    AND    NOVY    (FREDERICK    G.). 

PTOMAINS,  LEUCOMAINS,  TOXINS  AND  ANTITOXINS, 
or  the  Chemical  Factors  in  the  Causation  of  Disease.  Third  edition. 
In  one  12mo.  volume  of  603  pages. 


The  present  edition  has  been  not 
only  thoroughly  revised  throughout 
but  also  greatly  enlarged,  ample 
consideration  being  given  to  the  new 
subjects  of  toxins  and  antitoxins. — 
Tri-State  Medical  Journal. 


The  work  has  been  brought  down 
to  date,  and  will  be  found  entirely 
satisfactory. — Journal  of  the  Ameri- 
can Medical  Association. 

The  most  exhaustive  and  most  re- 
cent presentation  of  the  subject. — 
American  Jour,  of  the  Med.  Sciences. 

VISITING  LIST.  THE  MEDICAL  NEWS  VISITING  LIST  for  1900. 
Four  styles :  Weekly  (dated  for  30  patients);  Monthly  (undated  for 
120  patients  per  month) ;  Perpetual  (undated  for  30  patients  each 
week);  and  Perpetual  (undated  for  60  patients  each  week).  The  60- 
patient  book  consists  of  256  pages  of  assorted  blanks.  The  first  three 
styles  contain  32  pages  of  important  data,  thoroughly  revised,  and 
160  pages  of  assorted  blanks.  Each  in  one  volume,  price,  $1.25. 
With  thumb-letter  index  for  quick  use,  25  cents  extra.  Special  rates 
to  advance-paying  subscribers  to  THE  MEDICAL  NEWS  or  THE 
AMERICAN  JOURNAL  OF  THE  MEDICAL  SCIENCES,  or  both.  See  p.  32. 

WATSON  (THOMAS).  LECTURES  ON  THE  PRINCIPLES  AND 
PRACTICE  OF  PHYSIC.  A  new  American  from  the  fifth  and 
enlarged  English  edition,  with  additions  by  H.  HARTSHORNE,  M.  D. 
In  two  large  8vo.  vols.  of  1840  pp.,  with  190  cuts.  Cloth,  $9 ;  leather,  $11. 

WEST  (CHARLES).  LECTURES  ON  THE  DISEASES  PECULIAR 
TO  WOMEN.  Third  American  from  the  third  English  edition.  In 
one  octavo  volume  of  543  pages.  Cloth,  $3.75 ;  leather,  $4.75. 

ON  SOME  DISORDERS  OF  THE   NERVOUS  SYSTEM  IN 

CHILDHOOD.    In  one  small  12mo.  volume  of  127  pages.     Cloth,  $1. 

WHARTON  (HENRY  R.).  MINOR  SURGERY  AND  BANDAG- 
ING. New  (4th)  edition.  In  one  12mo.  volume  of  594  pages,  with 
502  engravings,  many  of  which  are  photographic.  $3.00,  net. 

Well  written,  conveniently  ar- 
ranged and  amply  illustrated.  It 
covers  the  field  so  fully  as  to  render 
it  a  valuable  text-book,  as  well  as  a 
work  of  ready  reference  for  sur- 
geons.— North  Amer.  Practitioner. 


The  part  devoted  to  bandaging  is 
perhaps  the  best  exposition  of  the 
subject  in  the  English  language.  It 
can  be  highly  commended  to  the 
student,  the  practitioner  and  the 
specialist.— The  Chicago  Medical 
Recorder. 


LEA  BHOTHEBS  &  Co.,  PHILADELPHIA  AND  NEW  YORK.     31 

WHITLA   (WILLIAM).      DICTIONARY    OF    TREATMENT,  OR 

THERAPEUTIC  INDEX.    Including  Medical  and  Surgical  Thera- 
peutics.    In  one  square  octavo  volume  of  917  pages.     Cloth,  $4. 

WILLIAMS  (DAWSON).  THE  MEDICAL  DISEASES  OF  CHIL- 
DREN. In  one  12mo.  volume  of  629  pages,  with  18  illustrations. 
Cloth,  $2.50,  net. 


The  descriptions  of  symptoms  are 
full,  and  the  treatment  recommended 
will  meet  general  approval.  Under 
each  disease  are  given  the  symptoms, 


diagnoses,  prognosis,  complications, 
and  treatment.  The  work  is  up  to 
date  in  every  sense. — The  Charlotte 
Medic&l  Journal. 


WILSON  (ERASMUS).  A  SYSTEM  OF  HUMAN  ANATOMY. 
A  new  and  revised  American  from  the  last  English  edition.  Illustrated 
with  397  engravings.  In  one  octavo  volume  of  616  pages.  Cloth,  $4 ; 
leather,  $5. 

WINCKEL  ON  PATHOLOGY  AND  TREATMENT  OF  CHILDBED. 

Translated  by  JAMES  R.  CHADWICK,  A.  M.,  M.  D.    With  additions 
by  the  Author.    In  one  octavo  volume  of  484  pages.     Cloth,  $4. 

WIPPERN  (A.  G.)  AND  BALLENGER  (W.  L.).  Shortly.  A 
POCKET  TEXT-BOOK  OF  DISEASES  OF  THE  EYE,  EAR, 
NOSE  AND  THROAT.  In  one  handsome  12mo.  volume  of  about 
400  pages,  with  many  illustrations.  Lea's  Series  of  Pocket  Text-books, 
edited  by  BERN  B.  GALLAUDET,  M.  D.  See  p.  17. 

WOHLER'S  OUTLINES  OF  ORGANIC  CHEMISTRY.  Translated 
from  the  eighth  German  edition,  by  IRA  REMSEN,  M.  D.  In  one 
12mo.  volume  of  550  pages.  Cloth,  $3. 

YEO  (I.  BURNEY).  FOOD  IN  HEALTH  AND  DISEASE.  New 
(2d)  edition.  In  one  12mo.  volume  of  592  pages,  with  4  engravings. 
Cloth,  $2.50.  See  Series  of  Clinical  Manuals,  page  26. 


We  doubt  whether  any  book  on 
dietetics  has  been  of  greater  or  more 
widespread  usefulness  than  has  this 
much-quoted  and  much-consulted 


work  of  Dr.  Yeo's.  The  value  of 
the  work  is  not  to  be  overestimated. 
— New  York  Medical  Journal. 


YOUNG  (JAMES  K.).    ORTHOPEDIC  SURGERY.    In    one    8vo. 
volume  of  475  pages,  with  286  illustrations.     Cloth,  $4 ;  leather,  $5. 

In  studying  the  different  chapters,  |  surgical  specialty  and  every  page 
one  is  impressed  with  the  thorough- 1  abounds  with  evidences  of  prac- 
ness  of  the  work.  The  illustrations  ticality.  It  is  the  clearest  and  most 


are  numerous — the  book  thoroughly 
practical — Medical  News. 

It  is  a  thorough,  a  very  compre- 
hensive work  upon  this  legitimate 


modern  work  upon  this  growing  de- 
partment of  surgery. — The  Chicago 
Clinical  Review. 


PERIODICALS. 


PROGRESSIVE  MEDICINE. 

A  Quarterly  Digest  of  New  Methods,  Discoveries,  and  Improvements 
in  the  Medical  and  Surgical  Sciences  by  Eminent  Authorities.  Edited  by 
Dr.  Hobart  Amory  Hare.  In  four  abundantly  illustrated,  cloth  bound, 
octavo  volumes,  of  400-500  pages  each,  issued  quarterly,  commencing 
March  1st,  1899.  Per  annum  (4  volumes),  $10.00  delivered. 


THE  MEDICAL  NEWS. 
Weekly,  $4.00  per  Annum. 

Each  number  contains  40  quarto  pages,  abundantly  illustrated.    A 
crisp,  fresh  weekly  professional  newspaper. 


THE  AMERICAN  JOURNAL  OF  THE  MEDICAL,  SCIENCES. 

Monthly,  j$4.0O  Per  Annum. 

Each  issue  contains  128  octavo  pages,  fully  illustrated.    The  most 
advanced  and  enterprising  American  exponent  of  scientific  medicine. 


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THE  MEDICAL  NEWS   POCKET  FORMULARY. 

Containing  over  1600  prescriptions  representing  the  latest  and  most  ap- 
proved methods  of  administering  remedial  agents.  Strongly  bound  in 
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COMBINATION    RATES: 

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THIS  BOOK  IS  DUE  ON  THE  LAST  DATE 
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WILL  INCREASE  TO  SO  CENTS  ON  THE  FOURTH 
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6    1934 


SEP  25  1935 


